drm/i915/dp: increase i2c-over-aux retry interval on AUX DEFER

There is no clear cut rules or specs for the retry interval, as there
are many factors that affect overall response time. Increase the
interval, and even more so on branch devices which may have limited i2c
bit rates.

Signed-off-by: Jani Nikula <[e-mail]>
Reference: https://bugs.freedesktop.org/show_bug.cgi?id=60263
Tested-by: Nicolas Suzor <[e-mail]>
Reviewed-by: Todd Previte <[e-mail]>
Cc: [e-mail]
Signed-off-by: Daniel Vetter <[e-mail]>

drivers/gpu/drm/i915/intel_dp.c

···
 			DRM_DEBUG_KMS("aux_ch native nack\n");
 			return -EREMOTEIO;
 		case AUX_NATIVE_REPLY_DEFER:
-			udelay(100);
+			/*
+			 * For now, just give more slack to branch devices. We
+			 * could check the DPCD for I2C bit rate capabilities,
+			 * and if available, adjust the interval. We could also
+			 * be more careful with DP-to-Legacy adapters where a
+			 * long legacy cable may force very low I2C bit rates.
+			 */
+			if (intel_dp->dpcd[DP_DOWNSTREAMPORT_PRESENT] &
+			    DP_DWN_STRM_PORT_PRESENT)
+				usleep_range(500, 600);
+			else
+				usleep_range(300, 400);
 			continue;
 		default:
 			DRM_ERROR("aux_ch invalid native reply 0x%02x\n",