[PATCH 06/10] drm/amd/pm: Treat zero vblank time as too short in si_dpm (v3)

[Timur Kristóf]

Some parts of the code base expect that MCLK switching is turned
off when the vblank time is set to zero.

According to pp_pm_compute_clocks the non-DC code has issues
with MCLK switching with refresh rates over 120 Hz.

v3:
Add code comment to explain this better.
Add an if statement instead of changing the switch_limit.

Fixes: 841686df9f7d ("drm/amdgpu: add SI DPM support (v4)")
Signed-off-by: Timur Kristóf <timur.kristof at gmail.com>
---
 drivers/gpu/drm/amd/pm/legacy-dpm/si_dpm.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/amd/pm/legacy-dpm/si_dpm.c b/drivers/gpu/drm/amd/pm/legacy-dpm/si_dpm.c
index fb008c5980d6..c11c4cc111df 100644
--- a/drivers/gpu/drm/amd/pm/legacy-dpm/si_dpm.c
+++ b/drivers/gpu/drm/amd/pm/legacy-dpm/si_dpm.c
@@ -3085,7 +3085,13 @@ static bool si_dpm_vblank_too_short(void *handle)
 	/* we never hit the non-gddr5 limit so disable it */
 	u32 switch_limit = adev->gmc.vram_type == AMDGPU_VRAM_TYPE_GDDR5 ? 450 : 0;
 
-	if (vblank_time < switch_limit)
+	/* Consider zero vblank time too short and disable MCLK switching.
+	 * Note that the vblank time is set to maximum when no displays are attached,
+	 * so we'll still enable MCLK switching in that case.
+	 */
+	if (vblank_time == 0)
+		return true;
+	else if (vblank_time < switch_limit)
 		return true;
 	else
 		return false;
-- 
2.51.0