Intel-kms in Linux-4.2rc causes regression due to dithering always on.

[Daniel Vetter]

On Thu, Aug 6, 2015 at 11:56 PM, Mario Kleiner
<mario.kleiner.de at gmail.com> wrote:
> Hi Daniel and all,
>
> since Linux 4.2 (tested with rc4), i think this commit
> d328c9d78d64ca11e744fe227096990430a88477
> "drm/i915: Select starting pipe bpp irrespective or the primary plane"
>
> causes trouble for me and my users, as tested on Intel HD Ironlake and Ivy
> Bridge with MiniDP->Singlelink-DVI adapter -> Measurement device.
>
> Afaics it causes dithering to always be enabled on a regular 8bpc
> framebuffer, even when outputting to a 8 bpc DVI-D output, and that
> dithering causes my display measurement equipment and other special display
> devices used for neuro-science and medical applications to fail. This
> equipment requires an identity passthrough of 8 bpc framebuffer pixels to
> the digital outputs, iow. dithering off.
>
> Log output on Linux 4.1 (good):
>
> Aug  1 06:39:26 twisty kernel: [  154.175394]
> [drm:connected_sink_compute_bpp] [CONNECTOR:35:HDMI-A-1] checking for sink
> bpp constrains
> Aug  1 06:39:26 twisty kernel: [  154.175396]
> [drm:intel_hdmi_compute_config] picking bpc to 8 for HDMI output
> Aug  1 06:39:26 twisty kernel: [  154.175397]
> [drm:intel_hdmi_compute_config] forcing pipe bpc to 24 for HDMI
> Aug  1 06:39:26 twisty kernel: [  154.175400] [drm:ironlake_check_fdi_lanes]
> checking fdi config on pipe A, lanes 1
> Aug  1 06:39:26 twisty kernel: [  154.175402]
> [drm:intel_modeset_pipe_config] plane bpp: 24, pipe bpp: 24, dithering: 0
> Aug  1 06:39:26 twisty kernel: [  154.175403] [drm:intel_dump_pipe_config]
> [CRTC:20][modeset] config for pipe A
> Aug  1 06:39:26 twisty kernel: [  154.175404] [drm:intel_dump_pipe_config]
> cpu_transcoder: A
> Aug  1 06:39:26 twisty kernel: [  154.175405] [drm:intel_dump_pipe_config]
> pipe bpp: 24, dithering: 0
>
> Log output on Linux 4.2-rc4 (bad):
>
> Aug  1 06:21:31 twisty kernel: [  200.924831]
> [drm:connected_sink_compute_bpp] [CONNECTOR:36:HDMI-A-1] checking for sink
> bpp constrains
> Aug  1 06:21:31 twisty kernel: [  200.924832]
> [drm:connected_sink_compute_bpp] clamping display bpp (was 36) to default
> limit of 24
> Aug  1 06:21:31 twisty kernel: [  200.924834]
> [drm:intel_hdmi_compute_config] picking bpc to 8 for HDMI output
> Aug  1 06:21:31 twisty kernel: [  200.924835]
> [drm:intel_hdmi_compute_config] forcing pipe bpc to 24 for HDMI
> Aug  1 06:21:31 twisty kernel: [  200.924838] [drm:ironlake_check_fdi_lanes]
> checking fdi config on pipe A, lanes 1
> Aug  1 06:21:31 twisty kernel: [  200.924840]
> [drm:intel_modeset_pipe_config] plane bpp: 36, pipe bpp: 24, dithering: 1
> Aug  1 06:21:31 twisty kernel: [  200.924841] [drm:intel_dump_pipe_config]
> [CRTC:21][modeset] config ffff880131a5c800 for pipe A
> Aug  1 06:21:31 twisty kernel: [  200.924842] [drm:intel_dump_pipe_config]
> cpu_transcoder: A
> Aug  1 06:21:31 twisty kernel: [  200.924843] [drm:intel_dump_pipe_config]
> pipe bpp: 24, dithering: 1
>
> Ideas what to do about this?

Well I somehow assumed the dither bit would be sane and not wreak
havoc with the lower bits when they would fit into the final bpc pipe
mode ... Can you confirm with your equipment that we seem to be doing
8bpc->6bpc dithering on the 8bpc sink?

If that's the case we simply limit to only ever dither when the sink
is 6bpc, and not in any other case.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch