[Bug 101528] RX460 Memory clock stays high until card is "used"

[bugzilla-daemon@freedesktop.org]

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https://bugs.freedesktop.org/show_bug.cgi?id=101528

--- Comment #2 from Alexander Tsoy <alexander@tsoy.me> ---
I've added printing of some debug info into smu7_hwmgr.c and here what I get
before GPU enters that state:

[  778.701843] AMDGPU: vblank_time_us: 630, switch_limit_us: 450          
[  778.707608] AMDGPU: vblank_time_us: 630, switch_limit_us: 450                
[  778.713379] AMDGPU: disable_mclk_switching: 0,
disable_mclk_switching_for_frame_lock: 0, info.display_count: 1,
smu7_vblank_too_short: 0, mode_info.refresh_rate: 60
[  778.748777] AMDGPU: vblank_time_us: 0, switch_limit_us: 450       
[  778.754361] AMDGPU: vblank_time_us: 0, switch_limit_us: 450  
[  778.759951] AMDGPU: disable_mclk_switching: 1,
disable_mclk_switching_for_frame_lock: 0, info.display_count: 1,
smu7_vblank_too_short: 1, mode_info.refresh_rate: 0

For some reason if refresh_rate = 0 then vblank_time_us = 0. Shouldn't the
latter be 0xffffffff instead? So I guess the following commit is the culprit:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=09be4a5219610a6fae3215d4f51f948d6f5d2609

and the following patch should fix (or workaround?) this issue:

- if (vblank_time_us < switch_limit_us)
+ if (vblank_time_us && (vblank_time_us < switch_limit_us))

After applying it:

[  409.588673] AMDGPU: vblank_time_us: 630, switch_limit_us: 450
[  409.594427] AMDGPU: vblank_time_us: 630, switch_limit_us: 450
[  409.600182] AMDGPU: disable_mclk_switching: 0,
disable_mclk_switching_for_frame_lock: 0, info.display_count: 1,
smu7_vblank_too_short: 0, mode_info.refresh_rate: 60
[  409.639750] AMDGPU: vblank_time_us: 0, switch_limit_us: 450
[  409.645321] AMDGPU: vblank_time_us: 0, switch_limit_us: 450
[  409.650917] AMDGPU: disable_mclk_switching: 0,
disable_mclk_switching_for_frame_lock: 0, info.display_count: 1,
smu7_vblank_too_short: 0, mode_info.refresh_rate: 0

$ cat /sys/class/drm/card0/device/pp_dpm_mclk
0: 150Mhz *
1: 300Mhz
2: 700Mhz
3: 1450Mhz

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