Why no spatial dithering to 10 bit depth on DCE?

Alex Deucher alexdeucher at gmail.com
Fri Feb 12 05:32:23 UTC 2021


On Thu, Feb 11, 2021 at 4:12 PM Mario Kleiner
<mario.kleiner.de at gmail.com> wrote:
>
> 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?

I suspect older DCE 8.x APUs have similar limitations.  Although it
may only be an issue with multiple monitors or something like that.  I
don't remember the details.  @Harry Wentland do you remember?

>
> 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.
>

Sounds good.

Alex

> -mario
>
>
>> Alex
>>
>>
>> > 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
>> >>
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