Why no spatial dithering to 10 bit depth on DCE?

Mario Kleiner mario.kleiner.de at gmail.com
Thu Feb 11 21:11:50 UTC 2021

On Wed, Feb 10, 2021 at 10:36 PM Alex Deucher <alexdeucher at gmail.com> wrote:

> On Wed, Feb 10, 2021 at 4:08 PM Mario Kleiner
> <mario.kleiner.de at gmail.com> wrote:
> >
> > Resending this one as well, in the hope of some clarification or
> background information.
> >
Thanks Alex.

I suspect this may have been a limitation from DCE11.0 (E.g.,
> carrizo/stoney APUs).  They had some bandwidth limitations with
> respect to high bit depth IIRC.  I suspect it should be fine on the
> relevant dGPUs.  The code was probably originally added for the APUs

That sounds as if it would make sense for me to try to submit a patch to
you that restricts this limitation to DCE 11.0 only?

All i can say is during my testing with DCE-8.3 over HDMI and DCE-11.2 over
DP under amdvlk with fp16 mode and ouptut_bpc set to 10 bpc, ie. dithering
down from 12 bpc to 10 bpc, i didn't notice any problems when hacking this
out, and photometer measurements showed good improvements of luminance
reproduction with dithering.

and just never updated or the changes were accidentally lost when we
> consolidated the DCE code.  Unfortunately, there are not a lot of apps
> that work properly in Linux with >8 bits per channel.
Mine does ;-). As does apparently the Kodi media player. And at least
Gnome/X11 works now, whereas KDE's Kwin/X11 used to work nicely, but
regressed. And amdvlk does have fp16 support now since a while ago, so
that's one way to get high precision without disturbing conventional
desktop apps. I'll probably look into Mesa's Vulkan/WSI for 10 bpc / fp16
sometime this year if nobody beats me to it.


> > Thanks,
> > -mario
> >
> > On Mon, Jan 25, 2021 at 3:56 AM Mario Kleiner <
> mario.kleiner.de at gmail.com> wrote:
> >>
> >> Hi Harry and Nicholas,
> >>
> >> I'm still on an extended quest to squeeze as much HDR out of Linux +
> your hw as possible, although the paid contract with Vesa has officially
> ended by now, and stumbled over this little conundrum:
> >>
> >> DC's set_spatial_dither() function (see link below) has this particular
> comment:
> >> "/* no 10bpc on DCE11*/" followed by code that skips dithering setup if
> the target output depth is 10 bpc:
> >>
> >>
> https://elixir.bootlin.com/linux/v5.11-rc4/source/drivers/gpu/drm/amd/display/dc/dce/dce_opp.c#L219
> >>
> >> I couldn't find any hint in the commit messages towards the reason, so
> why is that?
> >>
> >> This gets in the way if one has a HDR-10 monitor with 10 bit native
> output depth connected and wants to output a fp16 framebuffer and retain
> some of the > 10 bit linear precision by use of spatial dithering down to
> 10 bit. One only gets the same precision as a 10 bpc unorm fb. Also the
> routine is called for all DCE's, not only DCE11, so it affects all of them.
> >>
> >> The same restrictions don't exist in the old kms code for amdgpu-kms
> and radeon-kms. I added a mmio hack to Psychtoolbox to go behind the
> drivers back and hack the FMT_BIT_DEPTH_CONTROL register to use spatial
> dithering down to 10 bpc anyway to circumvent this limitation. My
> photometer measurements on fp16 framebuffers feeding into 10 bit output
> show that I get a nice looking response consistent with dithering to 10 bpc
> properly working on DCE. Also i don't see any visual artifacts in displayed
> pictures, so the hw seems to be just fine. This on DCE-11.2, and more
> lightly tested on DCE-8.3.
> >>
> >> So i wonder if this is some leftover from some hw bringup, or if there
> is a good reason for it being there? Maybe it could be removed or made more
> specific to some problematic asic?
> >>
> >> Thanks for any insights you could provide. Stay safe,
> >> -mario
> >>
> > _______________________________________________
> > amd-gfx mailing list
> > amd-gfx at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/amd-gfx
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.freedesktop.org/archives/amd-gfx/attachments/20210211/80b69d7a/attachment.htm>

More information about the amd-gfx mailing list