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

[Mario Kleiner]

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?

thanks,
-mario