drm/dp: Add an EDID quirk for the DPCD register access probe

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.

v2: Fix lockdep wrt. drm_dp_aux::hw_mutex when calling
    drm_dp_dpcd_set_probe_quirk() with a dependent lock already held.
v3: Add a helper for determining if DPCD probing is needed. (Jani)
v4:
- s/drm_dp_dpcd_set_probe_quirk/drm_dp_dpcd_set_probe (Jani)
- Fix documentation of drm_dp_dpcd_set_probe().
- Add comment at the end of internal quirk entries.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/20250609125556.109538-1-imre.deak@intel.com

2 files changed, 9 insertions, 0 deletions

diff --git a/include/drm/display/drm_dp_helper.h b/include/drm/display/drm_dp_helper.h
index e4ca35143ff9..3e35a68b2b41 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(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 77fd42608e70..3d1aecfec9b2 100644
--- a/include/drm/drm_edid.h
+++ b/include/drm/drm_edid.h
@@ -110,6 +110,9 @@ struct detailed_data_string {
 #define DRM_EDID_CVT_FLAGS_REDUCED_BLANKING  (1 << 4)
 
 enum drm_edid_quirk {
+	/* Do a dummy read before DPCD accesses, to prevent corruption. */
+	DRM_EDID_QUIRK_DP_DPCD_PROBE,
+
 	DRM_EDID_QUIRK_NUM,
 };