[Intel-gfx] [PATCH 1/1] drm/i915: Keep AUX block running when disabling DPMS for MST

[Lyude Paul]

While enabling/disabling DPMS before link training with MST hubs is
perfectly valid; unfortunately disabling DPMS results in some devices
disabling their AUX CH block as well. For SST this isn't as much of a
problem, but for MST we need to be able to continue handling aux
transactions even when none of the sinks are turned on since it's
possible for us to have a single atomic commit which results in
disabling each downstream sink, followed by subsequently re-enabling
each sink.

If we don't do this, we'll end up stalling any pending ESI interrupts
from the sink for up to 1ms. Unfortunately, dropping ESIs during this
timespan makes it so that link fallback retraining for MST (which I will
be submitting to the ML shortly) fails due to the channel EQ failure
interrupts potentially getting dropped. Additionally, when performing a
modeset that brings the hub status's link status from bad -> good having
ESIs disabled for that long causes us to miss the hub's response to us
trying to start link training as well.

Since any sink with MST is going to support DisplayPort 1.2 anyway, save
us the hassle of trying to wait until the sink comes back up and just
never shut the aux block down.

Changes since v2:
 - Fix patch name, no functional changes
Changes since v3:
 - acks, t-b's

Signed-off-by: Lyude Paul <lyude at redhat.com>
Acked-by: Dhinakaran Pandiyan <dhinakaran.pandiyan at intel.com>
Tested-by: Laura Abbott <labbott at redhat.com>
Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Cc: stable at vger.kernel.org
Fixes: ad260ab32a4d9 ("drm/i915/dp: Write to SET_POWER dpcd to enable MST hub.")
---
 drivers/gpu/drm/i915/intel_dp.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index 62f82c4298ac..0479c377981b 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -2589,11 +2589,13 @@ void intel_dp_sink_dpms(struct intel_dp *intel_dp, int mode)
 		return;
 
 	if (mode != DRM_MODE_DPMS_ON) {
+		unsigned char data = intel_dp->is_mst ?
+			DP_SET_POWER_D3_AUX_ON : DP_SET_POWER_D3;
+
 		if (downstream_hpd_needs_d0(intel_dp))
 			return;
 
-		ret = drm_dp_dpcd_writeb(&intel_dp->aux, DP_SET_POWER,
-					 DP_SET_POWER_D3);
+		ret = drm_dp_dpcd_writeb(&intel_dp->aux, DP_SET_POWER, data);
 	} else {
 		struct intel_lspcon *lspcon = dp_to_lspcon(intel_dp);
 
-- 
2.14.3