On Tue, 03 Jun 2025, Imre Deak imre.deak@intel.com wrote:

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 Signed-off-by: Imre Deak imre.deak@intel.com

Acked-by: Jani Nikula jani.nikula@intel.com

---

drivers/gpu/drm/display/drm_dp_helper.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c index f2a6559a27100..dc622c78db9d4 100644 --- a/drivers/gpu/drm/display/drm_dp_helper.c +++ b/drivers/gpu/drm/display/drm_dp_helper.c @@ -725,7 +725,7 @@ ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset, * monitor doesn't power down exactly after the throw away read. */ if (!aux->is_remote) {

• ret = drm_dp_dpcd_probe(aux, DP_DPCD_REV);
  

• ret = drm_dp_dpcd_probe(aux, DP_LANE0_1_STATUS);
  

  if (ret < 0) return ret; }