Re: [Linaro-mm-sig] CDF discussions at FOSDEM

On 01/29/2013 04:50 PM, Daniel Vetter wrote:

On Tue, Jan 29, 2013 at 3:19 PM, Daniel Vetter[off-list ref]  wrote:
Ok, in the interest of pre-heating the discussion a bit I've written down
my thoughts about display slave drivers. Adding a few more people and
lists to make sure I haven't missed anyone ...

Cheers, Daniel
--
Display Slaves
=======

A highly biased quick analysis from Daniel Vetter.

And here is my biased version as one of the initiators of the idea of CDF.

I work with ARM SoCs (ST-Ericsson) and mobile devices (DSI/DPI panels). 
Of course some of these have the "PC" type of encoder devices like HDMI 
and eDP or even VGA. But from what I have seen most of these encoders 
are used by few different SoCs(GPUs?). And using these type of encoders 
was quite straight forward from DRM encoders. My goal was to get some 
common code of all the "mobile" panel encoders or "display module driver 
IC"s as some call them. Instead of tens of drivers (my assumption) you 
now have hundreds of drivers often using MIPI DSI/DPI/DBI or some 
similar interface. And lots of new come each year. There are probably 
more panel types than there are products on the market, since most 
products use more than one type of panel on the same product to secure 
sourcing for mass production (note multiple panels use same driver IC).
So that was the initial goal, to cover all of these, which most are 
maintained per SoC/CPU out of kernel.org. If HDMI/DP etc fits in this 
framework, then that is just a nice bonus.
I just wanted to give my history so we are not trying to include to many 
different types of encoders without an actual need. Maybe the I2C drm 
stuff is good enough for that type of encoders. But again, it would be 
nice with one suit that fits all ...
I also like the idea to start out small. But if no support is added 
initially for the mobile panel types. Then I think it will be hard to 
get all vendors to start pushing those drivers, because the benefit of 
doing so would be small. But maybe the CDF work with Linaro and Laurent 
could just be a second step of adding the necessary details to your 
really simple baseline. And I also favor the helpers over framework 
approach but I miss a big piece which is the ops for panel drivers to 
call back to display controller (the video source stuff).
Some inline comments below.

A quick discussion about the issues surrounding some common framework for
display slaves like panels, hdmi/DP/whatever encoders, ... Since these external
chips are very often reused accross different SoCs, it would be beneficial to
share slave driver code between different chipset drivers.

Caveat Emperor!
---------------

Current output types and slave encoders already have to deal with a pletoria of
special cases and strange features. To avoid ending up with something not
suitable for everyone, we should look at what's all supported already and how we
could possibly deal with those things:

- audio embedded into the display stream (hdmi/dp). x86 platforms with the HD
   Audio framework rely on ELD and forwarding certain events as interrupts
   through the hw between the display and audio side ...

I would assume any driver handling audio/video/cec like HDMI would hook 
itself up as an mfd device. And one of those exposed functions would be 
the CDF part. Instead of pushing everything into the "display parts". At 
least that is sort of what we do today and it keeps the audio, cec and 
display parts nicely separated.

Yes, it is a mess. But we have managed to hide that below a simple panel 
API similar to CDF/omap so far.

- dpms is 4 states in drm, even more in fbdev afaict, but real hw only supports
   on/off nowadays ... how should/do we care?

Agreed, they should all really go away unless someone find a valid use case.

Are you saying drm modes doesn't support this as of today? I have not 
used these types of modes in DRM yet. Maybe the common video mode 
patches is a good start.

Some standardized properties would be nice :). Whatever is not standard 
doesn't really matter.

One thing that is needed is all the meta data related to the 
control/data interface between display controller and encoder. Because 
this has to be unified per interface type like DSI/DBI so the same CDF 
driver can setup different display controllers. But I hope we could 
split the "CDF API" (panel ops) from the control/data bus API 
(host/source ops or CDF video source).

This is actually the place I wanted to start. With vendor specific panel 
drivers using common ops to access the bus (DSI/I2C/DBI etc). Then once 
we have a couple of panel drivers we could unify the API making them do 
their stuff (like the current CDF ops). Or even better, maybe these two 
could be made completely separate and worked on in parallel.

I prefer a slave only thing forwarded to the drm encoder which I assume 
would be the drm equivalent of the display slave. At least I have not 
seen any need to involve the display controller in hpd (which I assume 
you mean by master).

I'm not that familiar with the bios/uefi world. But on our SoCs we 
always have to show a splash screen from the boot loader (like bios, 
usually little kernel, uboot etc). And so all probing is done by 
bootloader and HW is running when kernel boot. And you are not allowed 
to disrupt it either because that would yield visual glitches during 
boot. So some way or the other the boot loader would need to transfer 
the state to the kernel or you would have to reverse engineer the state 
from hw at kernel probe.

This sounds more like a display controller feature than a display slave 
feature.

- PSR/dsi command mode and how the start/stop frame dance should be handled.

Again, a vital piece for the many mobile driver ICs. And I think we have 
several sources (STE, Renesas, TI, Samsung, ...) on how to do this and 
tested in many products. So I hope this could be an early step in the 
evolution.

Again, we have several samples of platforms already doing this stuff. So 
we should be able to get a draft pretty early. From my experience when 
to enable/disable video stream could vary between versions of the same 
display controller. So I think it could be pretty hairy to get a single 
solution for all. Instead I think we need to leave some room for the 
master/slave to decide when to enable/disable. And to be able to do this 
we should try to have pretty specific ops on the slave and master. I'm 
not sure prepare/modeset/commit is specific enough unless we document 
what is expected to be done by the slave in each of these.

- There's more I'm sure, gfx hw tends to be insane ...

Yes, and one is the chain of slaves issue that is "common" on mobile 
systems. One example I have is 
dispc->dsi->dsi2dsi-bridge->dsi2lvds-bridge->lvds-panel.
My proposal to hide this complexity in CDF was aggregate drivers. So 
from drm there will only be one master (dispc) and one slave (dsi2dsi). 
Then dsi2dsi will itself use another CDF/slave driver to talk to its 
slave. This way the top master (dispc) driver never have to care about 
this complexity. Whether this is possible to hide in practice we will 
see ...

Wishful Thinking
----------------

Ignoring reality, let's look at what the perfect display slave framework should
achieve to be useful:

- Should be simple to share code between different master drivers - display slave
   drivers tend to be boring assemblies of register definitions and banging the
   right magic values into them. Which also means that we should aim for a high
   level of unification so that using, understanding and debugging drivers is
   easy.

- Since we expect drivers to be simple, even little amounts of
   impedence-matching code can kill the benefits of the shared code. Furthermore
   it should be possible to extend drivers with whatever subset of the above
   feature list is required by the subsystem/driver using a slave driver. Again,
   without incurring unnecessary amounts of impendance matching. Ofc, not all
   users of slave drivers will be able to use all the crazy features.

This is also my fear. Which is why I wanted to start with one slave 
interface at a time. And maybe even have different "API"s for differnt 
type of panels. Like classic I2C encoders, DSI command mode "smart" 
panels, DSI video mode, DPI ... and then do another layer of helpers in 
drm encoders. That way a DSI command mode panel wouldn't have to be 
forced into the same shell as a I2C HDMI encoder as they are very 
different with very little overlap.

Reality Check
-------------

We already have tons of different slave encoder frameworks sprinkled all over
the kernel, which support different sets of crazy features and are used by
different. Furthermore each subsystem seems to have come up with it's own way to
describe metadata like display modes, all sorts of type enums, properties,
helper functions for special output types.

Conclusions:

- Throwing away and rewriting all the existing code seems unwise, but we'll
   likely need tons of existing drivers with the new framework.

- Unifying the metadata handling will be _really_ painful since it's deeply
   ingrained into each driver. Not unifying it otoh will lead to colossal amounts
   of impendance matching code.

- The union of all the slave features used by all the existing frameworks is
   impressive, but also highly non-overlapping. Likely everyone has his own
   utterly "must-have" feature.

Proposal
--------

I have to admit that I'm not too much in favour of the current CDF. It has a bit
of midlayer smell to it imo, and looks like it will make many of the mentioned
corner-case messy to enable. Also looking at things the proposed generic video
mode structure it seems to lack some features e.g. drm_mode already has. Which
does not include new insanity like 3d modes or some advanced infoframes stuff.

So instead I'll throw around a few ideas and principles:

- s/framework/helper library/ Yes, I really hate midlayers and just coming up
   with a different name seems to go a long way towards saner apis.

Me like, but I hope you agree to keep calling it CDF until it is merged. 
We could call it Common Display Frelpers if you like ;)

To also cover and be useful to mobile panels I suggest starting with 
on/off using a fixed mode initially. Because modeset is not used for 
most mobile panels (they only have one mode).

Guideline for I2C slave drivers maybe? Do we really want to enforce how 
drivers are implemented when it doesn't affect the API?
Also, I don't think it fits in general for slaves. Since DSI/DBI have 
not only registers but also operations you can execute using control 
interface.

Agreed, and I think we should rely on upper layers like drm as much as 
possible for locking.

Then, once we've gotten things of the ground and have some slave encoder drivers
which are actually shared between different subsystems/drivers/platforms or
whatever we can start to organically grow more common interfaces. Ime it's much
easier to simply extract decent interfaces after the fact than trying to come
up.

Now let's pour this into a more concrete form:

struct display_slave_ops {
         /* modeset ops, e.g. prepare/modset/commit from drm */
};

struct display_slave {
         struct display_slave_ops *ops;
         void *driver_private;
};

I think even just that will be worth a lot of flames to come up with a good and
agreeable interface for everyone. It'll probably satisfactory to no one though.

Then each subsystem adds it's own magic, e.g.

struct drm_encoder_slave {
         struct display_slave slave;

         /* everything else which is there already and not covered by the display
          * slave interface. */
};

I like the starting point. Hard to make it any more simple ;). But next 
step would probably follow quickly. I also like the idea to have current 
drivers aggregate the slave to make transition easier. CDF as it is now 
is an all or nothing API. And since you don't care how slaves interact 
with master (bus ops) I see the possibility still to separate "CDI 
device API" and "CDF bus API". Which would allow using DSI bus API for 
DSI panels and I2C bus API (or regmap) for I2C encoders instead of force 
use of the video source API in all slave drivers.

Other subsystems/drivers like DSS would embed the struct display_slave in their
own equivalent data-structure.

So now we have the little problem that we want to have one single _slave_ driver
codebase, but it should be able to support n different interfaces and
potentially even more ways to be initialized and set up. Here's my idea how this
could be tackled:

1. Smash everything into one driver file/directory.
2. Use a common driver structure which contains pointers/members for all
possible use-cases. For each interface the driver supports, it'll allocate the
same structure and put the pointer into foo->slave.driver_private. This way
different entry points from different interfaces could use the same internal
functions since all deal with the same structure.
3. Add whatever magic is required to set up the driver for different platforms.
E.g. and of match, drm_encoder_slave i2c match and some direct function to set
up hardcoded cases could all live in the same file.

Getting the kernel Kconfig stuff right will be fun, but we should get by with
adding tons more stub functions. That might mean that an of/devicetree platform
build carries around a bit of gunk for x86 vbt matching maybe, but imo that
shouldn't ever get out of hand size-wise.

Once we have a few such shared drivers in place, and even more important,
unified that part of the subsystem using them a bit, it should be painfully
obvious which is the next piece to extract into the common display slave library
interface. After all, they'll live right next to each another in the driver
sources ;-)

Eventually we should get into the real fun part like dsi bus support or command
mode/PSR ... Those advanced things probably need to be optional.

But imo the key part is that we aim for real unification in the users of
display_slave's, so internally convert over everything to the new structures.
That should also make code-sharing much easier, so that we could move existing
helper functions to the common display helper library.

What about drivers that are waiting for CDF to be pushed upstream 
instead of having to push another custom panel framework? I'm talking of 
my own KMS driver ... but maybe I could put most of it in staging and 
move relevant parts of DSI/DPI/HDMI panel drivers to "common" slave 
drivers ...

Bikesheds
---------

I.e. the boring details:

- Where to put slave drivers? I'll vote for anything which does not include
   drivers/video ;-)

drivers/video +1, drivers/gpu -1, who came up with putting KMS under 
drivers/gpu ;)

As suggested, start with on/off and static/fixed mode would help single 
resolution LCDs. Actually that is almost all that is needed for mobile 
panels and what I intended to get from CDF :)

- Naming the new interfaces. I'll have more asbestos suites on order ...

Until you get them. Would it make sense to reuse the encoder name from 
drm or is that to restrictive?

- Can we just copy the new "native" interface structs from drm, pls?

I hope you are not talking about the helper interfaces at least ;). But 
if CDF is going to be the new drm helpers of choice for 
encoder/connector parts. Then it sounds like CDF would replace most of 
the old helpers. It would be far to many layers with the old helpers 
too. And I think I recall Jesse wanting to deprecate/remove them too.
Hopefully we could have some generic encoder/connector helper 
implementations that only depend on CDF.

/BR
/Marcus