16 bpc fixed point (RGBA16) framebuffer support for core and AMD.

[Alex Deucher]

On Fri, Apr 16, 2021 at 12:29 PM Mario Kleiner
<mario.kleiner.de at gmail.com> wrote:
>
> Friendly ping to the AMD people. Nicholas, Harry, Alex, any feedback?
> Would be great to get this in sooner than later.
>

No objections from me.

Alex


> Thanks and have a nice weekend,
> -mario
>
> On Fri, Mar 19, 2021 at 10:03 PM Mario Kleiner
> <mario.kleiner.de at gmail.com> wrote:
> >
> > Hi,
> >
> > this patch series adds the fourcc's for 16 bit fixed point unorm
> > framebuffers to the core, and then an implementation for AMD gpu's
> > with DisplayCore.
> >
> > This is intended to allow for pageflipping to, and direct scanout of,
> > Vulkan swapchain images in the format VK_FORMAT_R16G16B16A16_UNORM.
> > I have patched AMD's GPUOpen amdvlk OSS driver to enable this format
> > for swapchains, mapping to DRM_FORMAT_XBGR16161616:
> > Link: https://github.com/kleinerm/pal/commit/a25d4802074b13a8d5f7edc96ae45469ecbac3c4
> >
> > My main motivation for this is squeezing every bit of precision
> > out of the hardware for scientific and medical research applications,
> > where fp16 in the unorm range is limited to ~11 bpc effective linear
> > precision in the upper half [0.5;1.0] of the unorm range, although
> > the hardware could do at least 12 bpc.
> >
> > It has been successfully tested on AMD RavenRidge (DCN-1), and with
> > Polaris11 (DCE-11.2). Up to two displays were active on RavenRidge
> > (DP 2560x1440 at 144Hz + HDMI 2560x1440 at 120Hz), the maximum supported
> > on my hw, both running at 10 bpc DP output depth.
> >
> > Up to three displays were active on the Polaris (DP 2560x1440 at 144Hz +
> > 2560x1440 at 100Hz USB-C DP-altMode-to-HDMI converter + eDP 2880x1800 at 60Hz
> > Apple Retina panel), all running at 10 bpc output depth.
> >
> > No malfunctions, visual artifacts or other oddities were observed
> > (apart from an adventureous mess of cables and adapters on my desk),
> > suggesting it works.
> >
> > I used my automatic photometer measurement procedure to verify the
> > effective output precision of 10 bpc DP native signal + spatial
> > dithering in the gpu as enabled by the amdgpu driver. Results show
> > the expected 12 bpc precision i hoped for -- the current upper limit
> > for AMD display hw afaik.
> >
> > So it seems to work in the way i hoped :).
> >
> > Some open questions wrt. AMD DC, to be addressed in this patch series, or follow up
> > patches if neccessary:
> >
> > - For the atomic check for plane scaling, the current patch will
> > apply the same hw limits as for other rgb fixed point fb's, e.g.,
> > for 8 bpc rgb8. Is this correct? Or would we need to use the fp16
> > limits, because this is also a 64 bpp format? Or something new
> > entirely?
> >
> > - I haven't added the new fourcc to the DCC tables yet. Should i?
> >
> > - I had to change an assert for DCE to allow 36bpp linebuffers (patch 4/5).
> > It looks to me as if that assert was inconsistent with other places
> > in the driver where COLOR_DEPTH121212 is supported, and looking at
> > the code, the change seems harmless. At least on DCE-11.2 the change
> > didn't cause any noticeable (by myself) or measurable (by my equipment)
> > problems on any of the 3 connected displays.
> >
> > - Related to that change, while i needed to increase lb pixelsize to 36bpp
> > to get > 10 bpc effective precision on DCN, i didn't need to do that
> > on DCE. Also no change of lb pixelsize was needed on either DCN or DCe
> > to get > 10 bpc precision for fp16 framebuffers, so something seems to
> > behave differently for floating point 16 vs. fixed point 16. This all
> > seems to suggest one could leave lb pixelsize at the old 30 bpp value
> > on at least DCE-11.2 and still get the > 10 bpc precision if one wanted
> > to avoid the changes of patch 4/5.
> >
> > Thanks,
> > -mario
> >
> >
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