[PATCH 3/4] drm/dp: Add an EDID quirk for the DPCD register access probe

[Imre Deak <imre.deak@intel.com>]

Reading DPCD registers has side-effects and some of these can cause a
problem for instance during link training. Based on this it's better to
avoid the probing quirk done before each DPCD register read, limiting
this to the monitor which requires it. Add an EDID quirk for this. Leave
the quirk enabled by default, allowing it to be disabled after the
monitor is detected.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
---
 drivers/gpu/drm/display/drm_dp_helper.c | 15 ++++++++++++++-
 drivers/gpu/drm/drm_edid.c              |  3 +++
 include/drm/display/drm_dp_helper.h     |  6 ++++++
 include/drm/drm_edid.h                  |  3 ++-
 4 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
index dc622c78db9d4..4f2e919c6f7a6 100644
--- a/drivers/gpu/drm/display/drm_dp_helper.c
+++ b/drivers/gpu/drm/display/drm_dp_helper.c
@@ -691,6 +691,19 @@ void drm_dp_dpcd_set_powered(struct drm_dp_aux *aux, bool powered)
 }
 EXPORT_SYMBOL(drm_dp_dpcd_set_powered);
 
+/**
+ * drm_dp_dpcd_set_dpcd_probe_quirk() - Set whether a probing before DPCD access is done
+ * @aux: DisplayPort AUX channel
+ * @enable: Enable the probing if required
+ */
+void drm_dp_dpcd_set_probe_quirk(struct drm_dp_aux *aux, bool enable)
+{
+	mutex_lock(&aux->hw_mutex);
+	aux->dpcd_probe_disabled = !enable;
+	mutex_unlock(&aux->hw_mutex);
+}
+EXPORT_SYMBOL(drm_dp_dpcd_set_probe_quirk);
+
 /**
  * drm_dp_dpcd_read() - read a series of bytes from the DPCD
  * @aux: DisplayPort AUX channel (SST or MST)
@@ -724,7 +737,7 @@ ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset,
 	 * We just have to do it before any DPCD access and hope that the
 	 * monitor doesn't power down exactly after the throw away read.
 	 */
-	if (!aux->is_remote) {
+	if (!aux->is_remote && !aux->dpcd_probe_disabled) {
 		ret = drm_dp_dpcd_probe(aux, DP_LANE0_1_STATUS);
 		if (ret < 0)
 			return ret;
diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index e867315253493..9250b425ae230 100644
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -255,6 +255,9 @@ static const struct edid_quirk {
 };
 
 static const struct edid_quirk global_edid_quirk_list[] = {
+	/* HP ZR24w DP AUX DPCD access requires probing to prevent corruption. */
+	DRM_EDID_QUIRK(drm_edid_encode_panel_id('H', 'W', 'P', 0x2869), PANEL_NAME_ANY,
+		       BIT(DRM_EDID_QUIRK_DP_DPCD_PROBE)),
 };
 
 /*
diff --git a/include/drm/display/drm_dp_helper.h b/include/drm/display/drm_dp_helper.h
index e4ca35143ff96..75fa9548aefa0 100644
--- a/include/drm/display/drm_dp_helper.h
+++ b/include/drm/display/drm_dp_helper.h
@@ -523,10 +523,16 @@ struct drm_dp_aux {
 	 * @no_zero_sized: If the hw can't use zero sized transfers (NVIDIA)
 	 */
 	bool no_zero_sized;
+
+	/**
+	 * @dpcd_probe_disabled: If probing before a DPCD access is disabled.
+	 */
+	bool dpcd_probe_disabled;
 };
 
 int drm_dp_dpcd_probe(struct drm_dp_aux *aux, unsigned int offset);
 void drm_dp_dpcd_set_powered(struct drm_dp_aux *aux, bool powered);
+void drm_dp_dpcd_set_probe_quirk(struct drm_dp_aux *aux, bool enable);
 ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset,
 			 void *buffer, size_t size);
 ssize_t drm_dp_dpcd_write(struct drm_dp_aux *aux, unsigned int offset,
diff --git a/include/drm/drm_edid.h b/include/drm/drm_edid.h
index 3d8e168521c82..a878805c81d97 100644
--- a/include/drm/drm_edid.h
+++ b/include/drm/drm_edid.h
@@ -110,7 +110,8 @@ struct detailed_data_string {
 #define DRM_EDID_CVT_FLAGS_REDUCED_BLANKING  (1 << 4)
 
 enum drm_edid_quirk {
-	DRM_EDID_QUIRK_NONE,
+	/* Do a dummy read before DPCD accesses, to prevent corruption. */
+	DRM_EDID_QUIRK_DP_DPCD_PROBE,
 };
 
 struct detailed_data_monitor_range {
-- 
2.44.2