Re: [PATCH v3 1/2] dt-bindings: pwm: Add Xilinx AXI Timer

quoted

On 5/17/21 4:40 PM, Sean Anderson wrote:

On 5/17/21 4:28 AM, Michal Simek wrote:

On 5/14/21 7:13 PM, Sean Anderson wrote:

On 5/14/21 4:50 AM, Michal Simek wrote:

On 5/13/21 10:43 PM, Rob Herring wrote:
On Thu, May 13, 2021 at 10:28 AM Sean Anderson
[off-list ref] wrote:

On 5/13/21 10:33 AM, Sean Anderson wrote:
    >
    >
    > On 5/12/21 10:16 PM, Rob Herring wrote:
    >  > On Tue, May 11, 2021 at 03:12:37PM -0400, Sean Anderson
wrote:
    >  >> This adds a binding for the Xilinx LogiCORE IP AXI Timer.
This device is
    >  >> a "soft" block, so it has many parameters which would
not be
    >  >> configurable in most hardware. This binding is usually
automatically
    >  >> generated by Xilinx's tools, so the names and values of
some
properties
    >  >> must be kept as they are. Replacement properties have been
provided for
    >  >> new device trees.
    >  >
    >  > Because you have some tool generating properties is not a
reason we have
    >  > to accept them upstream.
    >
    > These properties are already in
arch/microblaze/boot/dts/system.dts and
    > in the devicetree supplied to Linux by qemu. Removing these
properties
    > will break existing setups, which I would like to avoid.
Already in use in upstream dts files is different than just
'automatically generated' by vendor tools.

    >
    >  > 'deprecated' is for what *we* have deprecated.
    >
    > Ok. I will remove that then.
    >
    >  >
    >  > In this case, I don't really see the point in defining new
properties
    >  > just to have bool.
    >
    > I don't either, but it was requested, by Michal...

Err, your comment on the original bindings was

    > Can't all these be boolean?
With no other context, yes that's what I would ask. Now you've given
me some context, between using the existing ones and 2 sets of
properties to maintain, I choose the former.

And Michal commented

    > I think in this case you should described what it is used by
current
    > driver in Microblaze and these options are required. The rest
are by
    > design optional.
    > If you want to change them to different value then current
binding
    > should be deprecated and have any transition time with code
alignment.
So that is what I tried to accomplish with this revision. I also
tried
allowing something like

           xlnx,one-timer-only = <0>; /* two timers */
           xlnx,one-timer-only = <1>; /* one timer  */
           xlnx,one-timer-only; /* one timer */
           /* property absent means two timers */

but I was unable to figure out how to express this with
json-schema. I
don't think it's the best design either...
json-schema would certainly let you, but generally we don't want
properties to have more than 1 type.
One thing is what it is in system.dts file which was committed in 2009
and there are just small alignments there. But none is really using it.
Maybe I should just delete it.
And this version was generated by Xilinx ancient tools at that time.
All
parameters there are fully describing HW and they are not changing.
Only
new one can be added.

   From the current microblaze code you can see which properties are
really
used.

reg
interrupts
xlnx,one-timer-only
clocks
clock-frequency
There is also an implicit dependency on xlnx,count-width. Several times
the existing driver assumes the counter width is 32, but this should
instead be discovered from the devicetree.
For me it is important what it is used now. Which is not
xlnx,count-width. That's why if you want to add it you can as optional
property.
At the very least we should sanity check it. E.g. check that it is 32
and return -EINVAL if it is not.
I have not a problem with it but make sure that the check is there only
when property is present not to break all current users.

It means from my point of view these should be listed in the binding.
clock-frequency is optional by code when clock is defined.

All other properties listed in system.dts are from my perspective
optional and that's how it should be.
Here is the situation as I understand it

* This device has existed for around 15 years (since 2006)
* Because it is a soft device, there are several configurable parameters
* Although all of these parameters must be known for a complete
   implementation of this device, some are unnecessary if onlu reduced
   functionality is needed.
* A de facto devicetree binding for this device has existed for at least
   12 years (since 2009), but likely for as long as the device itself
has
   existed. This binding has not changed substantially during this time.
note: IP itself is even much older.

* This binding is present in devicetrees from the Linux kernel, from
   qemu, in other existing systems, and in devicetrees generated by
   Xilinx's toolset.
Only from Linux. Qemu is trying to reuse the same properties but it can
also add own one. They are trying to be aligned as much as possible but
there are a lot of cases where Qemu requires much more information. (I
am not saying in this timer case but in general).

* Because the existing driver for this device does not implement all
   functionality for this device, not all properties in the devicetree
   binding are used. In fact, there is (as noted above) one property
   which should be in use but is not because the current driver
   (implicitly) does not support some hardware configurations.
* To support additional functionality, it is necessary to
   use hardware parameters which were not previously necessary.

Based on the above, we can classify the properties of this binding into
several categories.

* Those which are currently read by the driver.
   * compatible
   * reg
   * clocks
   * clock-frequency
   * interrupts
   * xlnx,one-timer-only

* Those which reflect hardware parameters which are currently explicitly
   or implicitly relied upon by the driver.
   * reg
   * clocks
   * clock-frequency
   * interrupts
   * xlnx,counter-width
   * xlnx,one-timer-only

* Those which are currently present in device trees.
   * compatible
   * reg
   * interrupts
   * clocks
   * clock-frequency
   * xlnx,count-width
   * xlnx,one-timer-only
   * xlnx,trig0-assert
   * xlnx,trig1-assert
   * xlnx,gen0-assert
   * xlnx,gen1-assert

When choosing what properties to use, we must consider what the impact
of our changes will be on not just the kernel but also on existing users
of this binding:
I don't think that this is valid. Rob is asking for adding #pwm-cells
which is purely Linux binding. We also don't know what properties are
used by others projects not just Linux or Qemu. Also required properties
in Linux doesn't need to be required in U-Boot for example even we are
trying to aligned all of them. Another case are others RTOSes, etc.
Here I do not see a way around this. Any way we do it we will need to
have some new binding. However, as noted below, adding a new binding for
configuration is easier than exposing properties in new ways.
please look below.

* To use properties currently present in device trees, we just need to
   modify the kernel driver.
* To add additional properties (such as e.g. '#pwm-cells'), we must
   modify the kernel driver. In addition, users who would like to use
   these new properties must add them to their device trees. This may be
   done in a mechanical way using e.g. overlays.
* To deprecate existing properties and introduce new properties to
   expose the same underlying hardware parameters, we must modify the
   kernel driver. However, this has a large impact on existing users.
   They must modify their tools to generate this information in a
   different format. When this information is generated by upstream
tools
   this may require updating a core part of their build system. For many
   projects, this may happen very infrequently because of the risk that
   such an upgrade will break things. Even if you suggest that Xilinx
can
   easily modify its tools to generate any sort of output, the time for
   this upgrade to be deployed/adopted may be significantly longer.
 From Xilinx perspective it would be ideal to use only properties which
fully describe HW in the form how they are generated today. They are
stable for a lot of years and as I said only new one are added.
But this alignment wasn't accepted long time ago and we have been asked
to start to align these properties with similar HW done by others.
And truth is that in a lot of cases there is clear 1:1 mapping and
generic properties can be simply use. This mapping ends in Xilinx device
tree generator.
Back to your point. Required properties are required by Linux driver
only. This driver is around for quite a long time where certain policies
haven't been setup/used/enforced (Microblaze is 2nd architecture which
started to use device tree).
We should create DT binding doc at that time but in 2009 it wasn't
standard practice. In 2007 Grant was adding support for Xilinx PPC
platform also without any DT binding document too.

That's why we need to review current unwritten DT binding based on code
requirements and look at it how to fix it (if needed) and then add PWM
support on the top of it.
If something needs to be deprecated, let's deprecate it and have
transition time for a year or so to adapt to it.

Rob knows much better than I how this should be handled.

Based on your list:
   * compatible
   * reg
   * clocks
   * clock-frequency
   * interrupts
   * xlnx,one-timer-only

all of these are required property in new DT binding.

xlnx,counter-width is optional if you want to use.
Again, for the existing driver this should at least be sanity-checked so
we fail noisily instead of buggily.
Till now for 10+ years none reported any issue with it that's why I
don't think this is a big problem.

This is the situation for the generate polarity as well. The driver will
work fine, but you will not have PWM output and there will be no
indication why. This is why I think we should start with supporting the
existing output. This is used to ensure the device will work as
configured. We can also add support for different properties exposing
the same information, but to support only new properties is not very
useful.
Binding can enforce properties to be required for PWM only and that's
totally okay. And because this is new feature and Rob don't need to like
that new properties they can be aligned. I also have preference to be as
much aligned with HW parameters but if they have to change then let's
change it and I will ensure that Xilinx DTG will generate them as they
were described to be aligned.

Thanks,
Michal

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