5.4-stable review patch. If anyone has any objections, please let me know.
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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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/gpu/drm/drm_dp_helper.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- 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_a * 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;