When enabling DC on the RX580 on Linux 4.15.0-rc2 I get a reduced color palette compared to having DC turned off.
Xorg drivers amdgpu 1.3.0
Attached you will find dmesg, xorg, and xrandr output with and without DC along with 2 pictures with and without dc for the results that I'm talking about.
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Without DC: Gnome shell overview
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DC: Gnome shell overview
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DC: Gnome shell shutdown dialog
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Without DC: Gnome shell shutdown dialog
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Without DC: xrandr --prop output
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Without DC: Xorg.log
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Without DC: dmesg -T
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DC: xrandr --prop output
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DC: dmesg -T
The EDID says 6 bits per channel, maybe DC doesn't enable dithering.
How do I establish whether that is what is happening or not? It sure looks like that's what's going on though.
Created attachment 136035 [details] [review]
Fix truncate programming
Does this help? Haven't had a chance to test it myself but this code looked obviously wrong.
I applied attachment 136035 [details] [review] to 4.15.0-rc2 but the problem remained with dc=1 on my rx580 I'm afraid.
I just upgraded to 4.16.0-rc1 and the problem is still there with dc=1
It appears that 4.16.0-rc7 fixed the issue on this laptop. I'll do a bit more testing but it seems on par with a system without dc now. Great stuff.
Thanks for testing. I'll set this to resolved. If you still see issues feel free to reopen.