Re: [PATCH 00/25] Exynos DRM: new life of IPP (Image Post Processing) subsystem

[Tobias Jakobi <tjakobi@math.uni-bielefeld.de>]

Hey Daniel,


Daniel Stone wrote:
> Hi,
> 
> On 12 November 2015 at 12:44, Tobias Jakobi
> <tjakobi@math.uni-bielefeld.de> wrote:
>> Daniel Stone wrote:
>>> On 10 November 2015 at 13:23, Marek Szyprowski <m.szyprowski@samsung.com> wrote:
>>>> This patch series introduces a new life into Exynos IPP (Image Post
>>>> Processing) subsystem by integrating it (transparently for userspace
>>>> applications) with Exynos DRM core plane management. This means that all
>>>> CRTC drivers transparently get support for standard features of IPP
>>>> subsystem like rotation and scaling.
>>>>
>>>> Support for features not supported natively by CRTC drivers is
>>>> implemented with a help of temporary framebuffers, where image data is
>>>> processed by IPP subsystem before performing the scanout by a CRTC driver.
>>>
>>> Hm, interesting. The RPi has a similar setup - VC4 can work either
>>> online (realtime scanout) or offline (mem2mem). Once the scene crosses
>>> a certain complexity boundary, it can no longer be composed in
>>> realtime and must fall back to mem2mem before it can be displayed.
>>>
>>> There was talk of having the fallback handled transparently in KMS for
>>> VC4 - similar to this - but the conclusion seemed to be that it was an
>>> inappropriate level of hidden complexity for KMS, and instead would
>>> best be handled by something like HWComposer directing it. Using HWC
>>> would then let you more intelligently split the scene from userspace
>>> (e.g. flatten some components but retain others as active planes).
>> I would be intererested in the performance implications of this
>> abstraction as well.
>>
>> I'd like to use the Exynos FIMC for CSC and scaling, but this operation
>> of course takes some time.
>>
>> I wonder how this interacts with page flipping. If I queue a pageflip
>> event with a buffer that needs to go through the IPP for display, where
>> does the delay caused by the operation factor it? If I understand this
>> correctly drmModePageFlip() still is going to return immediately, but I
>> might miss the next vblank period because the FIMC is still working on
>> the buffer.
> 
> Hmm, from my reading of the patches, this didn't affect page-flip
> timings. In the sync case, it would block until the buffer was
> actually displayed, and in the async case, the event would still be
> delivered at the right time. But you're right that it does introduce
> hugely variable timings, which can be a problem for userspace which
> tries to be intelligent.
I guess this is an issue then, since the FIMC is mainly used in the
context of converting video frames. And a good video player probably
wants to have tight control over the presentation of video frames, to
control A/V sychronization and so on.



> And even then potentially misleading from a
> performance point of view: if userspace can rotate natively (e.g. as
> part of a composition blit, or when rendering buffers in the first
> place), then we can skip the extra work from G2D.
> 
>> My problem here is that this abstraction would take too much control
>> from the user.
>>
>> Correct me if I have this wrong!
> 
> I believe that was the concern previously, yeah. :) That, and encoding
> these semantics in a user-visible way could potentially be dangerous.
I guess the local path transfers would still make sense though, but if I
understand this correctly this would affect FIMD.


With best wishes,
Tobias



> 
> Cheers,
> Daniel
>