[PATCH] drm/dp: Correctly mask DP_TRAINING_AUX_RD_INTERVAL values for DP 1.4

[matthew.s.atwood at intel.com]

From: Matt Atwood <matthew.s.atwood at intel.com>

DP_TRAINING_AUX_RD_INTERVAL with DP 1.3 spec changed bit scheme from 8
bits to 7 bits in DPCD 0x000e. The 8th bit describes a new feature, for
panels that use this new feature, this would cause a wait interval for
clock recovery of at least 512 ms, much higher then spec maximum of 16 ms.
This behavior is described in table 2-158 of DP 1.4 spec address 0000Eh.
To avoid breaking panels that are not spec compliant we now warn on
invalid values.

V2: commit title/message, masking all 7 bits, warn on out of spec values.

Signed-off-by: Matt Atwood <matthew.s.atwood at intel.com>
---
 drivers/gpu/drm/drm_dp_helper.c | 18 ++++++++++++++----
 include/drm/drm_dp_helper.h     |  1 +
 2 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index adf79be..a718ccc 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -119,18 +119,28 @@ u8 drm_dp_get_adjust_request_pre_emphasis(const u8 link_status[DP_LINK_STATUS_SI
 EXPORT_SYMBOL(drm_dp_get_adjust_request_pre_emphasis);
 
 void drm_dp_link_train_clock_recovery_delay(const u8 dpcd[DP_RECEIVER_CAP_SIZE]) {
-	if (dpcd[DP_TRAINING_AUX_RD_INTERVAL] == 0)
+	int rd_interval = dpcd[DP_TRAINING_AUX_RD_INTERVAL] & DP_TRAINING_AUX_RD_MASK;
+
+	if (rd_interval > 4)
+		DRM_DEBUG_KMS("AUX interval %d, out of range (max 4)", rd_interval);
+
+	if (rd_interval == 0)
 		udelay(100);
 	else
-		mdelay(dpcd[DP_TRAINING_AUX_RD_INTERVAL] * 4);
+		mdelay(rd_interval * 4);
 }
 EXPORT_SYMBOL(drm_dp_link_train_clock_recovery_delay);
 
 void drm_dp_link_train_channel_eq_delay(const u8 dpcd[DP_RECEIVER_CAP_SIZE]) {
-	if (dpcd[DP_TRAINING_AUX_RD_INTERVAL] == 0)
+	int rd_interval = dpcd[DP_TRAINING_AUX_RD_INTERVAL] & DP_TRAINING_AUX_RD_MASK;
+
+	if (rd_interval > 4)
+		DRM_DEBUG_KMS("AUX interval %d, out of range (max 4)", rd_interval);
+
+	if (rd_interval == 0)
 		udelay(400);
 	else
-		mdelay(dpcd[DP_TRAINING_AUX_RD_INTERVAL] * 4);
+		mdelay(rd_interval * 4);
 }
 EXPORT_SYMBOL(drm_dp_link_train_channel_eq_delay);
 
diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
index da58a42..f80acf1 100644
--- a/include/drm/drm_dp_helper.h
+++ b/include/drm/drm_dp_helper.h
@@ -118,6 +118,7 @@
 # define DP_DPCD_DISPLAY_CONTROL_CAPABLE     (1 << 3) /* edp v1.2 or higher */
 
 #define DP_TRAINING_AUX_RD_INTERVAL         0x00e   /* XXX 1.2? */
+# define DP_TRAINING_AUX_RD_MASK            0x7F     /* 1.3 */
 
 #define DP_ADAPTER_CAP			    0x00f   /* 1.2 */
 # define DP_FORCE_LOAD_SENSE_CAP	    (1 << 0)
-- 
2.7.4