Hello I'm inquiring about the amdgpu driver and temporal dithering. Let me give you a quick overview of the relevant parts of my setup. Monitor: BenQ GW2760HS (27" VA panel, native 8-bit, no FRC) connected via DVI GPU: ASUS R9 270X OS: Debian 8.x Jessie, also use Ubuntu MATE 16.04 It seems when using the above setup, despite the monitor being 8-bit I notice on dark grey and gradient images that the pixels are unstable and are "moving" with vertical banding, a telltale sign of dithering in my experience. I know there is supposedly a way to disable dithering on the older fglrx driver. Unfortunately as fglrx doesn't work with the new Xorg ABI which newer Linux distributions use I am unable to test it. https://forums.guru3d.com/threads/how-to-disable-dithering-in-linux.387362/ I cloned the kernel source from this repository and edited some of the dce_* files under drivers/gpu/drm/amd/amdgpu where it appears there are case statements that control the dithering done by the GPU (just an educated case), so said case statements do nothing instead of setting dithering registers and managed to recompile the kernel. Unfortunately that didn't seem to disable the temporal dithering so either I've patched the wrong functions for my GPU or something else. Frankly I wonder if AMD cards dither by default independent of the running graphics driver as I think I saw the dithering when I was running Ubuntu MATE on their grey colored boot screen (Plymouth). I'll try and a get a microscope and/or hand lens to try and see the LCD subpixels later. https://i.ytimg.com/vi/fANsyzPcXyM/maxresdefault.jpg ``` +++ b/drivers/gpu/drm/amd/amdgpu/dce_v6_0.c @@ -467,28 +467,6 @@ static void dce_v6_0_program_fmt(struct drm_encoder *encoder) switch (bpc) { - case 6: - if (dither == AMDGPU_FMT_DITHER_ENABLE) - /* XXX sort out optimal dither settings */ - tmp |= (FMT_BIT_DEPTH_CONTROL__FMT_FRAME_RANDOM_ENABLE_MASK | - FMT_BIT_DEPTH_CONTROL__FMT_HIGHPASS_RANDOM_ENABLE_MASK | - FMT_BIT_DEPTH_CONTROL__FMT_SPATIAL_DITHER_EN_MASK); - else - tmp |= FMT_BIT_DEPTH_CONTROL__FMT_TRUNCATE_EN_MASK; - break; ``` [sic] ``` default: /* not needed */ break; ``` My next goal is to figure out how to do live kernel debugging with another physical computer to see what functions are being hit in regards to dithering and color depth (as I could have edited a function that's not called in the case of the R9 270X I have). I'm actually working on a similar project with Macbook Pro's, OSX and dithering. I have 2x 2015 Macbook Pro's and are working on reverse engineering the AMD driver KEXT'S to try and disable dithering on OSX as it almost always happens with an external display. I have live debugging setup with lldb over a LAN with one computer being the host and another being the target, seems like that's not trivial to do under Linux. I know the motherboard inside my desktop has several COM port headers so I might be able to do kernel debugging of the GPU driver with the serial port and another computer. Something to look into. As for why I am doing this. First of all I am a photographer (one of many hobbies) and like having accurate colors in my workflow, second of all I used to get bad headaches from certain visual stimuli due to nervous system issues. To keep a long story short I've largely gotten that taken care of but I'm working on disabling dithering to ensure it's not something contributing to eye fatiguing. Third of all, it's been an interesting challenge for myself. Many thanks -- JTL Website: https://jtl.pw Email: jtl at teamclassified.ca (other contact methods available on request)
