[PATCH 5.4.y] drm/dp: Change AUX DPCD probe address from DPCD_REV to LANE0_1_STATUS

[Sasha Levin <sashal@kernel.org>]

From: Imre Deak <imre.deak@intel.com>

[ Upstream commit a40c5d727b8111b5db424a1e43e14a1dcce1e77f ]

Reading DPCD registers has side-effects in general. In particular
accessing registers outside of the link training register range
(0x102-0x106, 0x202-0x207, 0x200c-0x200f, 0x2216) is explicitly
forbidden by the DP v2.1 Standard, see

3.6.5.1 DPTX AUX Transaction Handling Mandates
3.6.7.4 128b/132b DP Link Layer LTTPR Link Training Mandates

Based on my tests, accessing the DPCD_REV register during the link
training of an UHBR TBT DP tunnel sink leads to link training failures.

Solve the above by using the DP_LANE0_1_STATUS (0x202) register for the
DPCD register access quirk.

Cc: <stable@vger.kernel.org>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/20250605082850.65136-2-imre.deak@intel.com
[ Call to drm_dp_dpcd_access() instead of drm_dp_dpcd_probe() ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/gpu/drm/drm_dp_helper.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index ffc68d305afe..4eabef5b86d0 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -280,7 +280,7 @@ 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.
 	 */
-	ret = drm_dp_dpcd_access(aux, DP_AUX_NATIVE_READ, DP_DPCD_REV, buffer,
+	ret = drm_dp_dpcd_access(aux, DP_AUX_NATIVE_READ, DP_LANE0_1_STATUS, buffer,
 				 1);
 	if (ret != 1)
 		goto out;
-- 
2.50.1