Resending this one as well, in the hope of some clarification or background information.
Thanks, -mario On Mon, Jan 25, 2021 at 3:56 AM Mario Kleiner <[email protected]> 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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