[Nouveau] nouveau 30bpp / deep color status

Wed Feb 7 16:28:42 UTC 2018

On Sun, Feb 04, 2018 at 06:50:45PM -0500, Ilia Mirkin wrote:
> In case anyone's curious about 30bpp framebuffer support, here's the
> current status:
> 
> Kernel:
> 
> Ben and I have switched the code to using a 256-based LUT for Kepler+,
> and I've also written a patch to cause the addfb ioctl to use the
> proper format. You can pick this up at:
> 
> https://github.com/skeggsb/linux/commits/linux-4.16 (note the branch!)
> https://patchwork.freedesktop.org/patch/202322/
> 
> With these two, you should be able to use "X -depth 30" again on any
> G80+ GPU to bring up a screen (as you could in kernel 4.9 and
> earlier). However this still has some deficiencies, some of which I've
> addressed:
> 
> xf86-video-nouveau:
> 
> DRI3 was broken, and Xv was broken. Patches available at:
> 
> https://github.com/imirkin/xf86-video-nouveau/commits/master
> 
> mesa:
> 
> The NVIDIA hardware (pre-Kepler) can only do XBGR scanout. Further the
> nouveau KMS doesn't add XRGB scanout for Kepler+ (although it could).
> Mesa was only enabled for XRGB, so I've piped XBGR through all the
> same places:
> 
> https://github.com/imirkin/mesa/commits/30bpp
> 
> libdrm:
> 
> For testing, I added a modetest gradient pattern split horizontally.
> Top half is 10bpc, bottom half is 8bpc. This is useful for seeing
> whether you're really getting 10bpc, or if things are getting
> truncated along the way. Definitely hacky, but ... wasn't intending on
> upstreaming it anyways:
> 
> https://github.com/imirkin/drm/commit/9b8776f58448b5745675c3a7f5eb2735e3989441
> 
> -------------------------------------
> 
> Results with the patches (tested on a GK208B and a "deep color" TV over HDMI):
>  - modetest with a 10bpc gradient shows up smoother than an 8bpc
> gradient. However it's still dithered to 8bpc, not "real" 10bpc.
>  - things generally work in X -- dri2 and dri3, xv, and obviously
> regular X rendering / acceleration
>  - lots of X software can't handle 30bpp modes (mplayer hates it for
> xv and x11 rendering, aterm bails on shading the root pixmap, probably
> others)
> 
> I'm also told that with DP, it should actually send the higher-bpc
> data over the wire. With HDMI, we're still stuck at 24bpp for now
> (although the hardware can do 36bpp as well). This is why my gradient
> result above was still dithered.
> 
> Things to do - mostly nouveau specific, but probably some general
> infra needed too:
>  - Figure out how to properly expose the 1024-sized LUT

We have the properties in the kernel. Not sure if x11 could expose it
to clients somehow, or would we just have to interpolate the missing
bits in the ddx?

>  - Add fp16 scanout

i915 could do this as well. There was a patch to just add the fourcc
on account of gvt needing it for some Windows thing. IIRC I asked them
to actually implement it in i915 proper but no patch ever surfaced.

>  - Stop relying on the max bpc of the monitor/connector and make
> decisions based on the "effective" bpc (e.g. based on the
> currently-set fb format, take hdmi/dp into account, etc). This will
> also affect the max clock somehow. Perhaps there should be a way to
> force a connector to a certain bpc.

We used to look at the fb depth for the primary plane when picking the
output bpc, but that doesn't really work when you have multiple planes,
and you generally don't want to have to do a modeset to flip to a fb
with another format. So in the end we just chose to go for the max
bpc possible.

There are some potential issues with deep color though (crappy HDMI
cables, dongles etc.) so I suggested a property to allow the user
to limit it below a certain value. Problem is that IIRC the patch we
got was just adding it to i915, whereas we really want to put it
into the drm core so that everyone will implement the same thing.

>  - Add higher-bpc HDMI support

Bunch of interesting stuff in i915 to figure out the sink/dongle clock
limit etc. If someone else is going to implement HDMI deep color we
should perhaps look into lifting some of that stuff into some common
place.

>  - Add 10bpc dithering (only makes sense if >= 10bpc output is
> *actually* enabled first)
>  - Investigate YUV HDMI modes (esp since they can enable 4K at 60 on HDMI
> 1.4 hardware)

We have 4:2:0 in i915, and pretty close to having YCbCr 4:4:4 too.
The 4:4:4 thing would need some new properties though so that the user
can actually enable it. What we do with 4:2:0 is enable it automagically
when the display can't do RGB 4:4:4 for the given mode. But there's 
currently no way for the user to say that they prefer YCbCr 4:2:0 over
RGB 4:4:4.

>  - Test out Wayland compositors
>  - Teach xf86-video-modesetting about addfb2 or that nouveau's
> ordering is different.
> 
> I don't necessarily plan on working further on this, so if there are
> interested parties, they should definitely try to pick it up. I'll try
> to upstream all my changes though.
> 
> Cheers,
> 
>   -ilia
> _______________________________________________
> dri-devel mailing list
> dri-devel at lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel

-- 
Ville Syrjälä
Intel OTC