[PATCH] drm/i915/psr: Only allow PSR in LPSP mode on HSW non-ULT

[Ville Syrjala]

From: Ville Syrjälä <ville.syrjala at linux.intel.com>

On HSW non-ULT (or at least on Dell Latitude E6540) external displays
start to flicker when we enable PSR on the eDP. We observe a much higher
SR and PC6 residency than should be possible with an external display,
and indeen much higher than what we observe with eDP disabled and
only the external display enabled. Looks like the hardware is somehow
ignoring the fact that the external display is active during PSR.

I wasn't able to redproduce this on my HSW ULT machine, or BDW.
So either there's something specific about this particular laptop
(eg. some unknown firmware thing) or the issue is limited to just
non-ULT HSW systems. All known registers that could affect this
look perfectly reasonable on the affected machine.

As a workaround let's unmask the LPSP event to prevent PSR entry
except while in LPSP mode (only pipe A + eDP active). This
will prevent PSR entry entirely when multiple pipes are active.
The one slight downside is that we now also prevent PSR entry
when driving eDP with pipe B or C, but I think that's a reasonable
tradeoff to avoid having to implement a more complex workaround.

Cc: stable at vger.kernel.org
Fixes: 783d8b80871f ("drm/i915/psr: Re-enable PSR1 on hsw/bdw")
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/10092
Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
---
 drivers/gpu/drm/i915/display/intel_psr.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_psr.c b/drivers/gpu/drm/i915/display/intel_psr.c
index 696d5d32ca9d..1010b8c405df 100644
--- a/drivers/gpu/drm/i915/display/intel_psr.c
+++ b/drivers/gpu/drm/i915/display/intel_psr.c
@@ -1544,8 +1544,18 @@ static void intel_psr_enable_source(struct intel_dp *intel_dp,
 	 * can rely on frontbuffer tracking.
 	 */
 	mask = EDP_PSR_DEBUG_MASK_MEMUP |
-	       EDP_PSR_DEBUG_MASK_HPD |
-	       EDP_PSR_DEBUG_MASK_LPSP;
+	       EDP_PSR_DEBUG_MASK_HPD;
+
+	/*
+	 * For some unknown reason on HSW non-ULT (or at least on
+	 * Dell Latitude E6540) external displays start to flicker
+	 * when PSR is enabled on the eDP. SR/PC6 residency is much
+	 * higher than should be possible with an external display.
+	 * As a workaround leave LPSP unmasked to prevent PSR entry
+	 * when external displays are active.
+	 */
+	if (DISPLAY_VER(dev_priv) >= 8 || IS_HASWELL_ULT(dev_priv))
+		mask |= EDP_PSR_DEBUG_MASK_LPSP;
 
 	if (DISPLAY_VER(dev_priv) < 20)
 		mask |= EDP_PSR_DEBUG_MASK_MAX_SLEEP;
-- 
2.41.0