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
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