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SubjectRe: Proposal for a low-level Linux display framework
Hi Rob,

(CC'ing linux-media, as I believe this is very on-topic)

On Sunday 18 September 2011 18:14:26 Rob Clark wrote:
> On Sat, Sep 17, 2011 at 6:12 PM, Laurent Pinchart wrote:
> > 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.
> I think we need to differentiate between V4L camera, and display..
> MC and subdev stuff clearly seem to be the way to go for complex
> camera / imaging subsystems. But that is a very different problem
> domain from GPU+display. We need to stop blurring the two topics.

I would agree with you if we were only talking about GPU, but display is
broader than that. Many hardware available today have complex display
pipelines with "deep tunneling" between other IP blocks (such as the camera
subsystem) and the display. Configuration of such pipelines isn't specific to

> > 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]).
> But I do think that overlays need to be part of the DRM/KMS interface,
> simply because flipping still needs to be synchronized w/ the GPU. I
> have some experience using V4L for display, and this is one (of
> several) broken aspects of that.

I agree that DRM/KMS must be used to address needs specific to display
hardware, but I don't think *all* display needs are specific to the display.

> > 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.
> I guess in theory it would be possible to let MC iterate the
> plane->crtc->encoder->connector topology.. I'm not entirely sure what
> benefit that would bring, other than change for the sake of change.

The MC API has been designed to expose pipeline topologies to userspace. In
the plane->crtc->encoder->connector case, DRM/KMS is probably enough. However,
many pipelines can't be described so simply. Reinventing the wheel doesn't
look like the best solution to me.

> V4L and DRM are very different APIs designed to solves very different
> problems. The KMS / mode-setting part may look somewhat similar to
> something you can express w/ a camera-like graph of nodes. But the
> memory management is very different. And display updates (like page
> flipping) need to be synchronized w/ GPU rendering.. etc. Trying to
> fit V4L here, just seems like trying to force a square peg in a round
> hole. You'd have to end up morphing V4L so much that in the end it
> looks like DRM. And that might not be the right thing for cameras.
> So V4L for camera, DRM for gpu/display. Those are the two APIs we need.

That's why I'm not advocating replacing DRM with V4L :-)

As explained in my previous mail, V4L and DRM started as monolithic APIs to
solve different needs. We now realize that they're actually made (or should be
made) of several sub-APIs. In the V4L case, that's pipeline discovery,
pipeline setup, format configuration (at the pad level in the pipeline,
including cropping, scaling and composing), controls (at the entity and/or pad
level in the pipeline), memory management and stream control (there are a
couple of other tasks we can add, but that's the basic idea). Some of those
tasks need to be performed for display hardware as well, and I believe we
should standardize a cross-subsystem (DRM, FB and V4L) API there. All the
display-specific needs that DRM has been designed to handle should continue to
be handled by DRM, I have no doubt about that.

To summarize my point, I don't want to fit V4L in DRM, but I would like to
find out which needs are common between V4L and DRM, and see if we can share
an API there.


Laurent Pinchart

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