Hello,
On 2015-11-12 15:46, Daniel Stone wrote:
On 12 November 2015 at 12:44, Tobias Jakobi
<tjakobi@xxxxxxxxxxxxxxxxxxxxx> wrote:
Daniel Stone wrote:
On 10 November 2015 at 13:23, Marek Szyprowski <m.szyprowski@xxxxxxxxxxx> 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. 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.
Page flip events are delivered to userspace at the right time. You are right
that there will be some delay between scheduling a buffer for display
and the
moment it gets displayed by hardware, but imho good application should sync
audio/video to the vblank events not the moment of scheduling a buffer. So
this delay should not influence on the final quality of displayed
The only problem I see, especially when color space conversion will be
added,
is how to tell generic application that some modes are preferred / not
preferred, so application would prefer native modes which are faster. On the
other hand application should be aware of the fact that hw scaling is
usually
faster / less power demanding than cpu scaling, so it is better to use such
mode with additional processing instead of doing that work with the cpu.
On the other hand Exynos hardware also provides so called LOCAL PATH feature
for image processing, in case of which no temporary buffer is needed. This
mode should not introduce a delay. Implementing it is on my TODO list.
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 believe that having this feature is quite beneficial for generic
applications
(like weston for example). It is especially very useful for video overlay
display, where scaling, rotation and colorspace conversion are typical
use-cases. An alternative would be to introduce some generic API for a frame
buffer conversions.
Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland
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