DRM device memory writeback (Mali-DP)

[Brian Starkey]

Hi Daniel,

Thanks for taking a look.

(+Cc Laurent)

On Fri, Jul 15, 2016 at 09:33:34AM +0200, Daniel Vetter wrote:
>On Thu, Jul 14, 2016 at 06:03:40PM +0100, Brian Starkey wrote:
>> Hi,
>>
>> The Mali-DP display processors have a memory-writeback engine which
>> can write the result of the composition (CRTC output) to a memory
>> buffer in a variety of formats.
>>
>> We're looking for feedback/suggestions on how to expose this in the
>> mali-dp DRM kernel driver - possibly via V4L2.
>>
>> We've got a few use cases where writeback is useful:
>>    - testing, to check the displayed image
>
>This might or might not need a separate interface. There are efforts to
>make the intel kms validation tests in i-g-t generic (well under way
>already), and part of that is creating a generic infrastructure to capture
>display CRCs for functional tests (still in progress).
>
>But it might be better if userspace abstracts between full readback and
>display CRC, assuming we can make full writeback cross-vendor enough for
>that use-case.
>

I'd lean towards the userspace abstraction.
Encumbering a simple CRC interface with all the complexity of
full-writeback (size, scaling, pixel format, multi-planar etc.) sounds
a bit unnecessary.

Of course, if v4l2 isn't going to be the cross-vendor full-writeback
implementation, then we need to be aiming to use whatever _is_ in
the mali-dp driver.

>>    - screen recording
>>    - wireless display (e.g. Miracast)
>>    - dual-display clone mode
>>    - memory-to-memory composition
>> Note that the HW is capable of writing one of the input planes instead
>> of the CRTC output, but we've no good use-case for wanting to expose
>> that.
>>
>> In our Android ADF driver[1] we exposed the memory write engine as
>> part of the ADF device using ADF's "MEMORY" interface type. DRM/KMS
>> doesn't have any similar support for memory output from CRTCs, but we
>> want to expose the functionality in the mainline Mali-DP DRM driver.
>>
>> A previous discussion on the topic went towards exposing the
>> memory-write engine via V4L2[2].
>>
>> I'm thinking to userspace this would look like two distinct devices:
>>    - A DRM KMS display controller.
>>    - A V4L2 video source.
>> They'd both exist in the same kernel driver.
>> A V4L2 client can queue up (CAPTURE) buffers in the normal way, and
>> the DRM driver would see if there's a buffer to dequeue every time a
>> new modeset is received via the DRM API - if so, it can configure the
>> HW to dump into it (one-shot operation).
>>
>> An implication of this is that if the screen is actively displaying a
>> static scene and the V4L2 client queues up a buffer, it won't get
>> filled until the DRM scene changes. This seems best, otherwise the
>> V4L2 driver has to change the HW configuration out-of-band from the
>> DRM device which sounds horribly racy.
>>
>> One further complication is scaling. Our HW has a scaler which can
>> tasked with either scaling an input plane or the buffer being written
>> to memory, but not both at the same time. This means we need to
>> arbitrate the scaler between the DRM device (scaling input planes) and
>> the V4L2 device (scaling output buffers).
>>
>> I think the simplest approach here is to allow V4L2 to "claim" the
>> scaler by setting the image size (VIDIOC_S_FMT) to something other
>> than the CRTC's current resolution. After that, any attempt to use the
>> scaler on an input plane via DRM should fail atomic_check().
>
>That's perfectly fine atomic_check behaviour. Only trouble is that the v4l
>locking must integrate into the drm locking, but that should be doable.
>Worst case you must shadow all v4l locks with a wait/wound
>drm_modeset_lock to avoid deadlocks (since you could try to grab locks
>from either end).
>

Yes, I haven't looked at the details of the locking but I'm hoping
it's manageable.

>> If the V4L2 client goes away or sets the image size to the CRTC's
>> native resolution, then the DRM device is allowed to use the scaler.
>> I don't know if/how the DRM device should communicate to userspace
>> that the scaler is or isn't available for use.
>>
>> Any thoughts on this approach?
>> Is it acceptable to both V4L2 and DRM folks?
>
>For streaming a V4L2 capture device seems like the right interface. But if
>you want to use writeback in your compositor you must know which atomic
>kms update results in which frame, since if you don't you can't use that
>composited buffer for the next frame reliable.
>
>For that case I think a drm-only solution would be better, to make sure
>you can do an atomic update and writeback in one step. v4l seems to grow
>an atomic api of its own, but I don't think we'll have one spanning
>subsystems anytime soon.
>

I've been thinking about this from the point of view of a HWComposer
implementation and I think the hybrid DRM-V4L2 device would work OK.
However it depends on the behaviour I mentioned above:

>> if the screen is actively displaying a
>> static scene and the V4L2 client queues up a buffer, it won't get
>> filled until the DRM scene changes.

V4L2 buffer queues are FIFO, so as long as the compositor queues only
one V4L2 buffer per atomic update, there's no ambiguity.
In the most simplistic case the compositor would alternate between:
  - Queue V4L2 buffer
  - DRM atomic update
... and dequeue either in the same thread or a different one. As long
as the compositor keeps track of how many buffers it has queued and
how many atomic updates it's made, it doesn't really matter.

We'd probably be looking to add in V4L2 asynchronous dequeue using
fences for synchronisation, but that's a separate issue.

>For the kms-only interface the idea was to add a property on the crtc
>where you can attach a writeback drm_framebuffer. Extending that idea to
>the drm->v4l case we could create special drm_framebuffer objects
>representing a v4l sink, and attach them to the same property. That would
>also solve the problem of getting some agreement on buffer metadata
>between v4l and drm side.
>

I think a drm_framebuffer on its own wouldn't be enough to handle our
scaling case - at that point it starts to look more like a plane.
However, if "special" CRTC sinks became a thing it could allow us to
chain our writeback output to another CRTC's input (via memory)
without a trip through userspace, which would be nice.

>Laurent had some poc patches a while ago for this, he's definitely the one
>to ping.

I've had a little look at: https://patchwork.kernel.org/patch/6026611/
It looks pretty similar to what I was hoping to do.

We have the advantage of being able to update the display scene and
writeback buffer together atomically in hardware, and to write-back in
a one-shot mode. This lets us make the DRM update queue and V4L2
buffer queues advance in lock-step.

>-Daniel

Thanks,
-Brian
>
>>
>> Thanks for your time,
>>
>> -Brian
>>
>> [1] http://malideveloper.arm.com/resources/drivers/open-source-mali-dp-adf-kernel-device-drivers/
>> [2] https://people.freedesktop.org/~cbrill/dri-log/?channel=dri-devel&date=2016-05-04
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>
>-- 
>Daniel Vetter
>Software Engineer, Intel Corporation
>http://blog.ffwll.ch
>