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
