drm/i915: Add a DP1.2 compatible way to read LTTPR capabilities

At least some DELL monitors (P2715Q) with DPCD_REV 1.2 return corrupted
DPCD register values when reading from the 0xF0000- LTTPR range with an
AUX transaction block size bigger than 1. The DP standard requires 0 to
be returned - as for any other reserved/invalid addresses - but these
monitors return the DPCD_REV register value repeated in each byte of the
read buffer. This will in turn corrupt the values returned by the LTTPRs
between the source and the monitor: LTTPRs must adjust the values they
read from the downstream DPRX, for instance right-shift/init the
downstream DP_PHY_REPEATER_CNT value. Since the value returned by the
monitor's DPRX is non-zero the adjusted values will be corrupt.

Reading the LTTPR registers one-by-one instead of reading all of them
with a single AUX transfer works around the issue.

According to the DP standard's 0xF0000 register description:
"LTTPR-related registers at DPCD Addresses F0000h through F02FFh are
valid only for DPCD r1.4 (or higher)." While it's unclear if DPCD r1.4
refers to the DPCD_REV or to the
LT_TUNABLE_PHY_REPEATER_FIELD_DATA_STRUCTURE_REV register (tickets filed
at the VESA site to clarify this haven't been addressed), one
possibility is that it's a restriction due to non-compliant monitors
described above. Disabling the non-transparent LTTPR mode for all such
monitors is not a viable solution: the transparent LTTPR mode has its
own issue causing link training failures and this would affect a lot of
monitors in use with DPCD_REV < 1.4. Instead this patch works around
the problem by reading the LTTPR common and PHY cap registers one-by-one
for any monitor with a DPCD_REV < 1.4.

The standard requires the DPCD capabilities to be read after the LTTPR
common capabilities are read, so re-read the DPCD capabilities after
the LTTPR common and PHY caps were read out.

v2:
- Use for instead of a while loop. (Ville)
- Add to code comment the monitor model with the problem.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/4531
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220322143844.42616-1-imre.deak@intel.com

1 files changed, 2 insertions, 0 deletions

diff --git a/include/drm/dp/drm_dp_helper.h b/include/drm/dp/drm_dp_helper.h
index 69487bd8ed56..8c0d22faad9a 100644
--- a/include/drm/dp/drm_dp_helper.h
+++ b/include/drm/dp/drm_dp_helper.h
@@ -2150,8 +2150,10 @@ bool drm_dp_read_sink_count_cap(struct drm_connector *connector,
 int drm_dp_read_sink_count(struct drm_dp_aux *aux);
 
 int drm_dp_read_lttpr_common_caps(struct drm_dp_aux *aux,
+				  const u8 dpcd[DP_RECEIVER_CAP_SIZE],
 				  u8 caps[DP_LTTPR_COMMON_CAP_SIZE]);
 int drm_dp_read_lttpr_phy_caps(struct drm_dp_aux *aux,
+			       const u8 dpcd[DP_RECEIVER_CAP_SIZE],
 			       enum drm_dp_phy dp_phy,
 			       u8 caps[DP_LTTPR_PHY_CAP_SIZE]);
 int drm_dp_lttpr_count(const u8 cap[DP_LTTPR_COMMON_CAP_SIZE]);