On Thu, 9 May 2019 11:09:26 +0200
"H. Nikolaus Schaller" [off-list ref] wrote:

Hi Jonathan,
sorry for the delayed response, but I have to focus on other topics now.

No worries. I have the same 'problem'!  Hence nothing yet from my side
on an alternative.

Just some small comments came to my mind to finish the discussion, mainly on the accelerometer in the lid case (see inline below).

BR and thanks,
Nikolaus

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Am 22.04.2019 um 16:20 schrieb Jonathan Cameron [off-list ref]:

On Mon, 15 Apr 2019 23:04:15 +0200
H. Nikolaus Schaller [off-list ref] wrote:
  

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Hi Jonathan,
we seem to have opposite goals and therefore quite different ideas what the right
solution is.  

Yup :)  Yikes this email has gotten complex.  Hopefully I haven't
lost track of the various points below.
  

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I come from user-space and the input side, find that input can handle abstract "acceleration input",
and some older chip drivers even provide that as their only interface. Therefore I want
to provide "acceleration input" in these cases where only iio capable drivers exist by
using the existing in-kernel-iio infrastructure. Nothing more.

You seem to come from the iio architecture and want to extend it to other APIs as
general as possible and input is just one of several of them.  

Yes, my target is to produce a subsystem that meets many (all would be nice)
requirements, including yours.  Whilst I'm happy to debate this for ever, I'm not
sure we are making any substantial progress.  As you mention below we
probably need to 'see the code' to drive the discussion forwards.
  

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Different goals usually lead to different solution architectures.  

Indeed, but in this case we have your proposal which is a subset of what
I am suggesting.  One architecture can fulfil both requirements.  

Not exactly. Yours always needs configuration in every case. My RFC works without
user-space config support for the most common cases. This user-space config must
be maintained and spread over all distributions. So we can never be sure that
if a user changes the distro that it still works.

Indeed, we disagree on this and will continue to do so.  My view
is that you will always need userspace policy anyway so might as
well apply the 'right' policy :)

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I'll leave it for the other thread, but Bastien has raised the case
(that I'd forgotten) that there already userspace stacks that are
capable of happily taking in both IIO and Input devices.  The confusion
here is they will now discover 'both' without the existing userspace
knowing that they are the same device.  We need to be very careful not
to break those userspace programs.

So this is an illustration of why the simplistic case doesn't work
'now'.
  

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Am 14.04.2019 um 13:40 schrieb Jonathan Cameron [off-list ref]:

On Mon, 8 Apr 2019 15:15:56 +0200
H. Nikolaus Schaller [off-list ref] wrote:
  

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Hi Jonathan,

I still do not fully understand what is worrying you here.    

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Do you worry about functionality, flexibility or resources or something else?    

Two main things:
1) Lack of generality of the approach. 
 This is a single use trick for input devices. Why does it make sense for
 input devices?    

No, it is not a trick...  

Bad choice of words. Sorry about that.

Any time we register a device as two types of device it is less than ideal.

If we had the true separation of IIO front end and back end then it
would be perfectly acceptable to 'only' have an input front end for
a given device.  That choice would still, in this sort of usecase,
have to be made from userspace. It's policy, not design.  If there are reasons
a particular device 'is input' then that mapping should be in DT or similar.
It's no different from knowing an ADC channel is wired to an analog
temperature sensor.  For example, you could build a joystick with
an accelerometer in the stick - then the usecase would be obvious!
Hence I would also argue that any dynamic interface should also support
a static path (DT or equivalent) for the cases where is really a
physical characteristic of the system!  Perhaps the Sony parts
fall into this category as well.

For a bit of historical background, there was a concerted effort
to produced a userspace stack for IIO for android.  
https://01.org/android-iio-sensors-hal

Unfortunately I think it died as a result of other moves in Intel on
one of their periodic shifts in direction.
  

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Why does it make sense for input devices?  

These are not just 'input' devices. They are accelerometers. One usecase
is to use them for input.  The exact same physical device is used for games
input uses and counting steps for example (actually a lot wider cases than
that, but this one is common in android devices).

Keep that in mind at all times. There are lots of usecases.
So we need a solution that does not result in problems for those
usecases.  We are not writing a subsytem targetting android use of
accelerometers. We are writing a subsystem addressing as many usecases
as we can of those devices.

Note that the original reason for IIO was to generalize a whole set of
proposed individual subsystems targeting particular sensor types. So
that is our focus - solutions that work for everyone.  This isn't
totally dissimilar from those discussions - at the time I wanted
to do a small focused ALS subsystem and got a resounding "no" from
Linus.  Generality matters a lot for the long term.
  

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a) because there is no alternative for accelerometer input devices and there are some
(older) accelerometer chips which only present themselves as input devices as soon
as the driver is compiled and the chip is found.  

Actually that is not accurate.  The vast majority of those older devices
that have had any attempt at mainlining are in IIO. AFAIK no accelerometer
driver has been merged to mainline input for many years. This is because,
amongst others, Dmitry has been of the view they didn't belong there for
a very long time.
  

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b) because input events define ACCEL input devices. But no Temperature or Voltage
input or generic ADC input.So there is no generalization of input devices beyond
keyboard, mouse, joystick, accelerometer and some others.  

That's not totally inaccurate, but the distinction in the other sensor cases
is that there is a clear 'additional' element that we can map in devicetree
which relates the IIO sensor channel to the input device.
Doesn't matter for the point of view of this discussion though.
  

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There are lots of other in kernel users and potential
 ones in the future.    

This bridge addition does not exclude any (additional) in-kernel use.  

No but it creates several problems:

1. Two ways to do the same thing. 
2. Two sets of code to maintain.
3. Confusion over what is the best way of doing it.
4. The known issues with multiple consumers (note my solution has
that problem as well!)

My job here is to maintain the code, which is why I push back on something
that makes that job harder.

When the next usecases comes along and someone says they want to map
all ADC channels to hwmon channels because that is the subystem that
they expect to measure voltages in, then I don't have a good argument
to stop them doing the same thing you have.

As a side note we have in the past had input drivers for gyroscopes and
magnetometers.  Why are accelerometers special?

I really don't see why we should treat accelerometers differently  

Not special. I just did not want to add them yet in the RFC phase.
The principle is the same.

hmm. I would argue that makes things even worse but there we are.
Now we have input interfaces for a wider set of drivers, many
of which aren't there for input at all.

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As this discussion runs on, I am increasingly convinced that there *must*
be a userspace policy control over whether an input device is
instantiated for a given accelerometer.   Once that is the
case then I cannot see a reason to treat it any differently from
other channel types.  

Well, I believe that we should avoid any user-space policy control that
can be avoided. At least those cases that can live without policy
control should not need to get one because there are other cases that
need one.

This is something we can't solve by discussion, of course.

The decision is that you are maintainer and I am just proposing an RFC.
So you have two votes and a veto right...

But maybe this can be fixed by proper defaults? I don't know.

Agreed, we aren't going to resolve this.  I need to at least find some
time to put an alternative on the table rather than just abstract
discussion.

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The ability to register additional IIO consumers like
 this is useful, lets make it useful to everyone.    

But not everyone and every iio device can and should be mappable to the "input" abstraction.
This does not make any sense to me.  

Absolutely.  It should not be.  I clearly didn't explain this well.

It should be mapped to a consumer.

One type of consumer is iio-input, another is iio-hmwon etc.
  

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For example, does it make sense to map a temperature sensor to accelerometer input? Or an
accelerometer to hwmon? This seems to be possible of your generalization unless I am missing something here.
If it is, it ignores that iio sensors are already grouped by what physical property they do measure.  

If people want to map crazy channels to crazy sensor outputs, why stop them?
(at this level of the interface).

This is a policy question for a userspace script.  Particular consumer drivers
could of course perform sanity checking and refuse to do anything if they
cannot sensibly use the channels.

Yes, the interface has this flexibility. Which is a good thing!  Take the example
of the gyroscope as input I used above.  If we want to add that support
in future to your driver (I have no idea if it actually makes sense)
then we can - without having to change the interface.
  

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2) To much generality of the specific usecase.  I don't want to put an Input
 interface on accelerometers where it makes no sense.    

I think you can just ignore the input interfaces in that case, if it was created.  

Bastien raised a case where this isn't true.
  

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The rule of it has
 2-3 axis so it must make sense isn't good enough to my mind.  How
 does userspace know which accelerometer to use (more and more devices have
 multiple?)    

In our approach user-space can make it known by udev rules based on /dev/input/event*
(not on iio but the input created for accelerometers). I think I mentioned that. This
comes for free for any device registering as input. So it is no additional code.  

Sorry, I'm lost.  What in there tells you to use 'this' interface rather than one
of the other N that were registered?  I'm not sure what information you
have available there.
  

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You could do something like looking at the location info from
 DT / ACPI in your driver and pick the 'best' but that's policy. Should be
 in userspace.  Sure you can just use the right input driver, but the moment
 we do that, we need aware userspace, if that's the case why not make it
 aware from the start.

Believe me I've been round this one a good few times and thought about it
a lot.    

That is fine and why we should discuss all the different aspects we have collected.
  

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I'll take a lot of convincing that this isn't a problem that
should be pushed into userspace.
  

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I think having them mapped always does not need much resources (except a handful of bytes
in memory and some µs during probing) unless the event device is opened and really used.
Only then it starts e.g. I2C traffic to read the sensors.    

The bytes don't really mater.    

Ok, good to know.
  

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The userspace ABI additions do.    

There are only new /dev/input/event devices with well defined ABI. This approach does
not invent anything new here, hence there are no ABI additions which we can break.  

But it does - we aren't talking general ABI, but ABI on specific
devices.  Sure, Android doesn't care - though you'd be amazed how much
individual android device developers will because we just added another
pile of tests to their CI.

An industrial sensor platform absolutely does.  They have to validate
those interfaces.  They can't just ignore them because they feel like
it because who knows if some future user will use them?

For another case, see Bastien's reply to the later thread.

Instantiating interfaces has testing costs, even when the are standard
interfaces.
  

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So it is just some unused file sitting around in /dev. Or 2 or could be even 100.
For devices which have no iio accelerometers configured, there will be no /dev/input
file. So we are discussing the rare case of devices with more than one or two accelerometers.    

Well they aren't exactly rare in IIO using systems ;)    

This is another thing where our experiences differ. What specific devices are you thinking
of? I am focussed on handhelds where the accelerometer (or two) is a way to do GUI input
depending on device orientation in space.  

Again, you are introducing this interface for everyone. Including lots of
'interesting' usecases.

I have worked with sensor platforms with accelerometers of different parts of humans,
We have people do bridge vibration measurement, flying UAVs and tracking the motion
of trucks.

Most are not huge numbers of accelerometers per node but don't rule out the
possibility.  It's normally limited by length of cables rather than anything
else so you used multiple nodes after a while each with their own set of sensors.

There are lots of plaforms out there that use multiple accelerometers in more
or less the same place to do very high dynamic range measurement (without losing
precision when things are nearly still).

Anyhow, it's not a particularly important point anyway!
  

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Now, on every system there are many interfaces and files that are not used because it makes
no sense to look at them. If I check on one of my systems, I find for example a lot of
/dev/tty and only a very small portion is used and generic distros have no issue with it.

There is even /dev/iio:device0 to /dev/iio:device5 representing the raw iio devices.
Not all of them are actively used, but they are simply there and can be scanned for.    

Agreed, in the ideal case we wouldn't have had that either, but we are
stuck with it.  The long term plan is to allow use of IIO backends without the
front end being there at all. Lots of SoC ADC users would prefer this. We are
stuck with the legacy intertwining fo the front end and back end of IIO so
this isn't as easy to do as I would like.    

Ah, ok. I think it is a similar discussion of hiding the serdev /dev/tty* if it is
used for accessing an embedded GPS or Bluetooth chip, for example.

But is this needed? I think it is not a problem if there are multiple consumers for
the same iio channel. Some in-kernel, some through /dev/iio:device* and maybe some
through /dev/input (which boils down to in-kernel).  

There are quite a few complexities around multiple consumers that we really haven't
solved.  Right now the cases that work are very much restricted.  I'd love
to tidy some of these up, but never enough time and all that.

It's not that relevant here, but in short a few of the issues are:
1) Interference over control settings.  - Two consumers need different filter
  settings/ sampling frequency / range.  How do we negotiate the choice and
  communicate it to the other consumers.  More complex questions such
  mediating choices of triggers.
2) One driver is doing polled reads, the other is doing interrupt driven.
  Most drivers prevent this combination because the polled reads can lead
  to unlimited delays on the interrupt driven path and hence break it.

The main driver for this separation was to present only the 'right' interface
to reduced people's validation costs etc.  People really do want to have the
option to strip back the userspace inteface.  Obviously these are the rare
people who would disable your config option, but the point of this was
that we actually would like to make even the IIO interface optional as
well but have a fair way to go before we can.
  

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So I do not see a resource problem if every accelerometer /dev/iio:device* gets
some companion /dev/input/event* for being used on demand - but only if this bridge
is configured at all.    

That argument does not apply. If we add a config option, distros will enable it.
So the vast majority of systems will ship with this turned on.  You cannot
use a config variable to control policy and expect it to be change by anyone
but a very very small subset of users.  So please drop the 'you can just not
build it argument'.    

This is not my point here. I mention this under the (now known to be wrong) assumption
that resources do care. I just want to state that kernels built for platforms where every
byte counts can be stripped down by disabling it. Others where resources are no concern
simply can map them all, even if not used.  

Agreed. A subset of users will just build without this.
  

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Userspace configuration changing is a lot easier if people actually care.
Sure, many distros will ship the same script to everyone.
  

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I think we need some deliberate userspace interaction to instantiate
one of these rather than 'always doing it'.      

My gut feeling is that this additional user-space interaction needs more resources and
adds a lot of complexity, independently of how it is done.    

Trivial resources and actually fairly trivial complexity.  Key thing is
it puts the burden on the users of this functionality to configure what they
want.    

Hm. No. My proposal does not need configuration which accelerometers should go where.  

Agreed. I was talking about my proposal here :)
  

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I assumethat input accelerometer users do not want to configure anything, like neither
a mouse or keyboard is to be configured to be useable (yes there are keymaps but that
is impossible to automate).  

The difference is a mouse is only really useful as a mouse and most of the time a keyboard
is a used only as a keyboard.  Here that's not true.
  

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They just want to be able to read device orientation in a device-independent scale.
Therefore my approach already takes the mount-matrix into account to hide sensor position
differences.  

And how does that work on the common case of a sensor in the lid of a laptop?
how do you know what angle the screen is at?    

Well, I am not aware of laptops where the sensor is in the lid because I am in the handhelds
business, but let's assume it is common.

I realized that if the sensor orientation is related to the lid position, while the reference
frame reported to user space is to be referenced to the lap or keyboard of the laptop, there does
not exist a static mount-matrix to describe it properly. So no driver can report that correctly.

Therefore, such a device needs a dynamic mount matrix, i.e. there should be a kernel driver that
reads out the lid angle sensor and modifies the mount-matrix of the accelerometer by some sin()/cos()
table.

This is where it is tricky.  There is no such lid angle sensor, that's what the accelerometer
is for. Typically you have a pair of accelerometers and need to know which is which. which one makes
sense for input use is non obvious.

The reason I raised this at all was to make the point that there is often a need for user
policy anyway. It's not a glorious world of things just magically working.

Well, you can delegate this to the user-space. But IMHO this is wrong layering. Every layer
put on top of the lower layers should abstract away implementation details so that the highest
layer has the most general interface.

In my view we have here this "protocol" stack (casting into the ISO 7-layer model):

	L7: application - user space
	L3: input - to get device orientation information
	L2: iio - to get raw data and mount-matrix
	L1: i2c, spi, usb, hdq, ... - to get bits from/to chips
	L0: chips, ...

My RFC mainly mangles the raw data reported from the iio level and the mount matrix into
device orientation information. I.e. it is a proposal for L3 implementation.

So fixing an issue of L2 (dynamic mount matrix for lid angle) in the user-space layer would be improper
layering.

So there are two possibilites:
a) make the mount-matrix dynamical as described above in L2
b) extend my RFC to handle this special case

I would argue that it becomes just another part of the userspace policy that
needs to be applied.  Sure the actual transform would be applied in kernel
but the decision on which accelerometer to use and if additional transforms
are needed, is one for userspace not the kernel.  Probably not that hard
to add as simple controls alongside a binding interface.

Thanks,

Jonathan

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One oddity to note here is that until very recently we deliberately didn't register
certain ACPI IDs because they confused userspace by reporting two accelerometers
without any info on which was in the lid.  Thankfully proper handling of that
is no being sorted.  It's still mostly a case of just deliberately ignoring one
of the sensors.
  

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And I think is even less flexible than "always doing it". Let me explain this claim.

For me, the kernel should present everything the hardware supports to user-space
in better digestable device files or APIs (without making any assumptions about the
user-space code).    

Agreed, we just have a different view on how this should be done. I want
it to be dynamic and extremely flexible, you want the easy way of just
putting a fixed set out all the time.
  

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Then, every user-space that will be installed can find out what the hardware supports
by looking at standard places.

E.g. it can scan for all mice and keyboards. And for all input accelerometers.    

Or, you an have the correct 'fairly trivial' userspace setup to scan for all
registered accelerometers and 'on demand' create the bindings to bring them up as
Input accelerometers if that is what makes sense for your platform.    

Why not scan for input accelerometers and leave it as an implementation detail that
the kernel does serve the physical chips through the iio infrastructure?  

If we could separate the IIO front end from the IIO backend I would agree that
would be another valid -userspace- policy.
  

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IMHO some user-spaces may already be scanning all */input/event* and check for
the device property INPUT_PROP_ACCELEROMETER.

This is a discussion mainly about proper encapsulation of lower level differences.
  

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If the kernel is hiding some chips and needs some initial user-space action before
presenting them all, this requires that the user-space has some a-priori knowledge
about which specific devices it should ask for.    

No more that it needs to know which accelerometer to use?    

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So it does not really need to scan
for them. Because it must already know. Obviously in some mapping table stored at
a well known location inside the rootfs image.    

No. Let me give some more details of how this would work.  It's really just
a more flexible version of what you have.

A distro, or individual user decides to put the relevant script in place for the
following:

1. Userspace detects a new accelerometer driver, via the standard methods (uevent)
2. Userspace looks to see if it has the required properties. Now this includes things
like detecting that it is the accelerometer in the lid of a laptop - if so do not
register it as an input device.  If it's in the keyboard then do register it.
3. Userspace script then creates the files in configfs
/sys/kernel/config/iio/maps/
(this interface needs appropriate definition)
Maybe...
/sys/kernel/config/iio/maps/iio_input/iio_device:X/accel_x, accel_y, etc
When done it writes to the bind file
/sys/kernel/config/iio/maps/iio_input/iio_device:X/bind
which instantiates the input driver.

This moves all of the policy decision into userspace, where it belongs.  If
we want to enable a particular accelerometer on a particular board because it
actually works better than the one the default policy says to use, then we can
do so.

The resulting infrastructure is much more general, because it lets us do the
same for any IIO consumer.  This input bridge is not a special case. It works
equally well for the existing hwmon bridge any would even let us do things
like provide the information from userspace that we have an analog accelerometer
wired up to an ADC on some hacker board.    

Ok, understood.

My approach triggers input uevents:

1. kernel detects a new iio accelerometer (looks like an analog accelerometer should be
  the DTS child of an iio adc and then iio should create an accelerometer and not a voltage
  channel)  

Yes ultimately it would be a child device that would be it's own IIO device. We
already have this for some gyroscopes.
  

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2. iio-bridge registers as input event
3. this triggers an uevent
4  an udev-rule can detect the properties and map it to some "speaking" name like
  /dev/input/main-accelerometer, /dev/input/lid-accelerometer etc. Or if the
  accelerometer is to be ignored, it does not get a "speaking" name at all.

The required udev rules are stored in user space and are of course user-space and application
specific. But this does not require to invent some new configfs stuff and special scripts
in user-space. Just install some udev rule at a well established location in file-system.  

I'm not sure there is any significant difference between you creating a mapping like
this an udev rule that creates the whole mapping.  Bit more to do perhaps but it's
nothing particularly special that I can see.  Sure there is new kernel support to be
done.
  

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Yes, this does not cover arbitrary mappings. But what are arbitrary mappings good
for? Your scheme seems to be able to map a light sensor to accelerometer input.
Does this "full matrix of everything is possible" really make sense?  

From a generic interface point of view - yes it absolutely does.

We define an interface that covers all usecases rather than a whole set of
separate ones that cover individual corner cases.  That way we don't have to
keep defining new interfaces.

The individual drivers can easily do validation of what they are provided with.
  

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I can't decide because I have no need for it. Others may have.

But another thought: does it interfere with this input-bridge? Probably no. You can
still add your configfs approach for general iio devices to e.g. hwmon mappings. Even
as an alternate method of creating input devices (enabled only if my input-bridge is
disabled).  

Yes see above.  Both approaches meet your requirement (I think anyway).
I do not want to see two long term solutions to the same problem.

I'm interested in a long term sustainable solution so I want to see
the generic one.
  

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This seems to make it impossible to develop a generic distro rootfs image - without
asking the user for manual configuration. And that where the kernel already knows
this (which iio accelerometers do exist for a specific piece of hardware).

This is why I believe a mechanism to instantiate only on demand isn't adding but
removing flexibility because it prevents copying a rootfs from one device to another.    

I disagree, see above.
  

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As I mentioned in V1, look at the possibility of a configfs based method
to build the map.  It's easy for userspace to work out what makes sense to
map in principle.  There may be some missing info that we also need to
look to expose.      

With a "may be missing" it is impossible to write code for it...
Can you please name which information is missing on the input accelerometer
API?    

See above. It's not the input accelerometer ABI, it's the missing ability
to instantiate IIO maps from user space.
  

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In general, userspace created channel maps would be very useful for
other things such as maker type boards where they can plug all sorts
of odd things into ADC channels for example.      

Ok, I understand, but this is a different problem where this iio-input-bridge is not
intended to be a solution. Generic ADCs are not input devices. Like SD cards are not
keyboards.

So we should not try to mix the idea of general mapping with this input-bridge for
input accelerometers.    

Yes we should. You are proposing a solution that is a subset of the larger
problem set.    

Yes, of course. Because I did not see or know about the general problem set.
And I still don't see a need for user-space controlled mapping for input-accelerometers.  

We are clearly going to differ on this.  Bastien gave one example for why
this is required.  There will be others.
  

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Why introduce a stop gap like this when we can do it correctly
and provide something useful for all those other use cases.

The only difference here is the uevent triggered script that creates those maps
for your particular usecase.    

Well, I am a friend of solving one problem after the other in smaller steps than
immediately aiming at a very general solution, which has side-effects of inventing
new stuff for things that would work without.  

That works in a world where you can drop the previous approach as part of your
generalization.  When you are playing with kernel / userspace ABI then it
doesn't. Ideally you have to figure out the extensible general solution at the
start because you are stuck maintaining the 'small steps' for many years to
come.  I don't want to perpetually 'have' to export all 3D accelerometers as
input devices, because we didn't have the ability to chose which should be
exported at some point in the past.
  

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BTW, there is a way to define additional mapping using udev rules which symlink the
/dev/input/event* paths to stable names like /dev/input/accelerometer.

This comes without additional code and is already provided by udev and the input system.

So in summary, I have not yet seen a convincing scenario where being able to dynamically
map iio channels to input devices seems beneficial.    

That is true for the narrow case you are talking about. I don't want to see that
narrow case solved in a fashion that effectively breaks solving it properly.    

How does it break your approach if added later? The more I think about it they are
not incompatible. It is just useless to apply both in parallel.  

The reality is that if we put one in first that will used for ever because there
will be devices out there using it.  Therefore we have to maintain both for
ever.
  

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If we add this, we have to export all accelerometers for ever under all circumstances
to userspace, because to remove it will break existing userspace.

If we stand back and work out if we can do the general solution now, we avoid
this problem.    

We get a different problem that we break existing user-space that simply wants to see
an /dev/input/accelerometer without doing more than an existing udev rule.  

I would love to say such a userspace doesn't exist, but reality is there are
all sorts of hideous things out there.  There are cases that deal with this
as an option of course (such as Bastien's sensor-proxy)

The number of devices that are supported under mainline as input accelerometers
is pretty small.  It's not a perfect world unfortunately but having to add a
small udev script is at least not a major break if we do cause it.  

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This driver simply collects the first 3 accelerometer channels as X, Y and Z.
If only 1 or 2 channels are available, they are used for X and Y only. Additional
channels are ignored.

Scaling is done automatically so that 1g is represented by value 256 and
range is assumed to be -511 .. +511 which gives a reasonable precision as an
input device.      

Why do we do this, rather than letting input deal with it?  Input is used
to widely differing scales IIRC      

Well, it can't be done differently... And what I call scale here is nothing more than
defining ABSMIN_ACC_VAL and ABSMAX_ACC_VAL.

We need to apply some scale since iio reports in (fractional) units of 1g, i.e. values
of magnitude 1.    

m/s^2 not g, but doesn't matter for the point of view of this discussion.    

My fault. The driver takes care of this in the scaling formula so that "input" reports
MAX/2 for 1g.
  

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These are not adaequate for input events which use integers. So we must
define some factor for iio_convert_raw_to_processed() to scale from raw value range
to int value range. We could report raw values but this would be an improper abstraction
from chip specific differences.    

Hmm. I can see we perhaps need some mapping, but is there a concept of standard scale
for existing input accelerometers?  How is this done to give for other input devices
such as touch screens?  I'd expect to see a separation between scale, and range.

  

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BTW: the range (and therefore the factor) is reported through the evdev driver to user-space
(evtest reports Min and Max as you can see in the example).

The most important thing is that this is a hardware independent definition. Every accelerometer
chip will report this range. So you can easily upgrade hardware or switch accelerometers
without touching user-space calibration. Like you can replace ethernet controller chips but
networking works the same with all of them.    

Agreed, it needs to be hardware independent by the time it hits userspace, but I would
have thought that scaling would be done in input, rather than IIO. It's hardly
a problem unique to our usecase!

Perhaps Dmitry can give some advice on this.    

Yes, that would be helpful.
  

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Hm. Is there an alternative to attach such private data to an struct iio_dev
allocated by someone else? I have not found one yet.

Or can I add some void *input_mapping; to struct iio_dev? Depending on
#if defined(CONFIG_IIO_INPUT_BRIDGE)?    

Yes, add a new element.    

Ok, works fine.

I already have found one case of iio accelerometer driver where it did make a problem
not using a special element.
  

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iio_input_find_accel_channel(indio_dev, chan, &numchans);
iio_input_register_device(indio_dev, chan, numchans);      

Well, that looks like it needs some temporary storage of dynamic size
and loop twice over channels for no functional benefit.    

Use fixed size. The worst that happens is we end up with it being
an entry larger that it needs to be.
  

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And handle the
special case of numchans == 0 (the proposed code simply does not call
iio_input_register_accel_channel and does not register anything).

So I'd prefer to follow the "KISS" principle and register single channels
instead of a set of channels.    

Well we disagree on this.  A singleton approach like used here
is to my mind not KISS.  I would rather see what is there then
act as two simple steps, rather than interleave two different
actions with a totally different path for the first channel found.
If there is only one channel you just built a load of infrastructure
that makes no sense.  If you scan first then you can know that
before building anything.    

Ok, this is more a matter of taste and resource requirements can probably
be neglected. I'll update the driver.

So in summary, I'll post a v3 that fixes some bugs of v2 (because we need
them fixed for our production systems as well).

Then it is up to you if you want to take this approach or want to write
a full version following your concept. Or if it is possible as I assume, we
can have both.  

Thanks. I think we need at some code for what I was proposing to discuss
much further. Unfortunately it may be a little while before I get time to
work on that.  Hopefully not too long though!

Jonathan
  

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BR and thanks,
Nikolaus