Hi,
For letting DRM clients to select output encoding:
Sink can support certain display timings with high output bit-depths using multiple output encodings, e.g. sink can support a particular timing with RGB 10-bit, YCbCr422 10-bit and YCbCr420 10-bit. So DRM client may want to select YCbCr422 10-bit over RBG 10-bit output to reduce the link bandwidth (and in turn reduce power/voltage). If DRM driver automatically selects output encoding then we are restricting DRM clients from making appropriate choice.
For selectable output color range:
Certain applications (typically graphics) usually rendered in full range while some applications (typically video) have limited range content. Since content can change dynamically, DRM driver does not have enough information to choose correct quantization. Only DRM client can correctly select which quantization to set (to preserve artist's intent).
For how to use selectable output encoding with Weston:
I was thinking that DRM should have separate property to list the encodings supported by sink and Weston will present this list to its client. Your idea to validate encodings using TEST_ONLY commit and present a list of timings along with encodings supported by particular timing seems better. Instead of validating all possible encodings, does it make sense to validate only those supported by sink? Irrespective of this we would anyway need some mechanism which will allow user to select particular encoding for a particular mode. I was thinking to allow this using new DRM property "Encoding". Do you have anything better in mind?
(Since I am using my Gmail Id, I feel I should mention that I work at Nvidia)
Thanks,
-Yogish
On Thu, May 28, 2020 at 6:18 PM Pekka Paalanen <ppaalanen@xxxxxxxxx> wrote:
On Thu, 28 May 2020 17:38:59 +0530
Yogish Kulkarni <yogishkulkarni@xxxxxxxxx> wrote:
> I am trying to find a way through Weston which will allow setting specific
> encoding at display output.
Hi,
why do *you* want to control that?
Why not let the driver always choose the highest possible encoding
given the video mode and hardware capability?
I can understand userspace wanting to know what it got, but why should
userspace be able to control it?
Would people want to pick the encoding first, and then go for the
highest possible video mode?
> Could you please elaborate on why it is best
> to let DRM driver automatically configure which encoding to choose rather
> than making it selectable by DRM client ? I am not able to find reference
> to past discussion about this. I was only able to find a proposed change -
> https://lists.freedesktop.org/archives/intel-gfx/2017-April/125451.html but
> am not able to find why it got rejected.
>
> Alternatively, is there existing way through which DRM clients can specify
> preference for output encoding ? Or currently it's all up to the DRM driver
> to choose what output encoding to use.
There must be some reason why userspace needs to be able to control it.
I'm also asking as a Weston maintainer, since I'm interested in how
this affects e.g. color reproduction or HDR support.
One thing that comes to my mind is using atomic TEST_ONLY commits to
probe all the possible video modes × encodings for presenting a list to
the user to choose from, if you have a display configuration GUI. E.g
with some TV use cases, maybe the user wants to avoid sub-sampling, use
the native resolution, but limit refresh rate to what's actually
possible. Or any other combination of the three.
Thanks,
pq
>
> Thanks,
> -Yogish
>
> On Thu, May 28, 2020 at 1:54 PM Daniel Vetter <daniel@xxxxxxxx> wrote:
>
> > On Thu, May 28, 2020 at 12:29:43PM +0530, Yogish Kulkarni wrote:
> > > For creating new source property, is it good to follow
> > > "drm_mode_create_hdmi_colorspace_property()" as an example ? It seems
> > that
> > > currently there is no standard DRM property which allows DRM client to
> > set
> > > a specific output encoding (like YUV420, YUV422 etc). Also, there is no
> > > standard property for letting client select YUV/RGB color range. I see
> > > there are two ways to introduce new properties, 1. do something like
> > > drm_mode_create_hdmi_colorspace_property 2. create custom property
> > similar
> > > to "Broadcast RGB". Is there opinion on which is a preferable way to
> > expose
> > > encoding and color rage selection property ?
> >
> > I guess first question is "why?" Thus far we've gone with the opinion that
> > automatically configuring output stuff as much as possible is best. What's
> > the use-case where the driver can't select this?
> > -Daniel
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