Proposal for a low-level Linux display framework

[Laurent Pinchart]

Hi everybody,

On Thursday 15 September 2011 20:39:21 Florian Tobias Schandinat wrote:
> On 09/15/2011 05:52 PM, Alex Deucher wrote:
> >
> > Please don't claim that the DRM developers do not want to cooperate.
> > I realize that people have strong opinions about existing APIs, put
> > there has been just as much, if not more obstinacy from the v4l and fb
> > people.
> 
> Well, I think it's too late to really fix this thing. We now have 3 APIs in
> the kernel that have to be kept. Probably the best we can do now is figure
> out how we can reduce code duplication and do extensions to those APIs in
> a way that they are compatible with each other or completely independent
> and can be used across the APIs.

Sorry for jumping late into the discussion. Let me try to shed some new light 
on this.

I've been thinking about the DRM/KMS/FB/V4L APIs overlap for quite some time 
now. All of them have their share of issues, historical nonsense and unique 
features. I don't think we can pick one of those APIs today and decide to drop 
the others, but we certainly need to make DRM, KMS, FB and V4L interoperable 
at various levels. The alternative is to keep ignoring each other and let the 
market decice. Thinking that the market could pick something like OpenMAX 
scares me, so I'd rather find a good compromise and move forward.

Disclaimer: My DRM/KMS knowledge isn't as good as my FB and V4L knowledge, so 
please feel free to correct my mistakes.

All our video-related APIs started as solutions to different problems. They 
all share an important feature: they assume that the devices they control is 
more or less monolithic. For that reason they expose a single device to 
userspace, and mix device configuration and data transfer on the same device 
node.

This shortcoming became painful in V4L a couple of years ago. When I started 
working on the OMAP3 ISP (camera) driver I realized that trying to configure a 
complex hardware pipeline without exposing its internals to userspace 
applications wouldn't be possible. DRM, KMS and FB ran into the exact same 
problem, just more recently, as showed by various RFCs ([1], [2]).

To fix this issue, the V4L community developed a new API called the Media 
Controller [3]. In a nutshell, the MC aims at

- exposing the device topology to userspace as an oriented graph of entities 
connected with links through pads

- controlling the device topology from userspace by enabling/disabling links

- giving userspace access to per-entity controls

- configuring formats at individual points in the pipeline from userspace.

The MC API solves the first two problems. The last two require help from V4L 
(which has been extended with new MC-aware ioctls), as MC is media-agnostic 
and can't thus configure video formats.

To support this, the V4L subsystem exposes an in-kernel API based around the 
concept of sub-devices. A single high-level hardware device is handled by 
multiple sub-devices, possibly controlled by different drivers. For instance, 
in the OMAP3-based N900 digital camera, the OMAP3 ISP is made of 8 sub-devices 
(all controlled by the OMAP3 ISP driver), and the two sensors, flash 
controller and lens controller all have their own sub-device, each of them 
controlled by its own driver.

All this infrastructure exposes the devices a the graph showed in [4] to 
applications, and the V4L sub-device API can be used to set formats at 
individual pads. This allows controlling scaling, cropping, composing and 
other video-related operations on the pipeline.

With the introduction of the media controller architecture, I now see V4L as 
being made of three parts.

1. The V4L video nodes streaming API, used to manage video buffers memory, map 
it to userspace, and control video streaming (and data transfers).

2. The V4L sub-devices API, used to control parameters on individual entities 
in the graph and configure formats.

3. The V4L video nodes formats and control API, used to perform the same tasks 
as the V4L sub-devices API for drivers that don't support the media controller 
API, or to provide support for pure V4L applications with drivers that support 
the media controller API.

V4L is made of those three parts, but I believe it helps to think about them 
individually. With today's (and tomorrow's) devices, DRM, KMS and FB are in a 
situation similar to what V4L experienced a couple of years ago. They need to 
give control of complex pipelines to userspace, and I believe this should be 
done by (logically) splitting DRM, KMS and FB into a pipeline control part and 
a data flow part, as we did with V4L.

Keeping the monolithic device model and handling pipeline control without 
exposing the pipeline topology would in my opinion be a mistake. Even if this 
could support today's hardware, I don't think it would be future-proof. I 
would rather see the DRM, KMS and FB topologies being exposed to applications 
by implementing the MC API in DRM, KMS and FB drivers. I'm working on a proof 
of concept for the FB sh_mobile_lcdc driver and will post patches soon. 
Something similar can be done for DRM and KMS.

This would leave us with the issue of controlling formats and other parameters 
on the pipelines. We could keep separate DRM, KMS, FB and V4L APIs for that, 
but would it really make sense ? I don't think so. Obviously I would be happy 
to use the V4L API, as we already have a working solution :-) I don't see that 
as being realistic though, we will probably need to create a central graphics-
related API here (possibly close to what we already have in V4L if it can 
fulfil everybody's needs).

To paraphrase Alan, in my semi-perfect world vision the MC API would be used 
to expose hardware pipelines to userspace, a common graphics API would be used 
to control parameters on the pipeline shared by DRM, KMS, FB and V4L, the 
individual APIs would control subsystem-specific parameters and DRM, KMS, FB 
and V4L would be implemented on top of this to manage memory, command queues 
and data transfers.

Am I looking too far in the future ?

[1] http://www.mail-archive.com/intel-gfx@lists.freedesktop.org/msg04421.html
[2] http://www.mail-archive.com/linux-samsung-
soc at vger.kernel.org/msg06292.html
[3] http://linuxtv.org/downloads/v4l-dvb-apis/media_common.html
[4] http://www.ideasonboard.org/media/omap3isp.ps

-- 
Regards,

Laurent Pinchart