[Intel-gfx] [PATCH v2] drm/i915/display: Reset message bus after each read/write operation

[Mika Kahola]

Every know and then we receive the following error when running
for example IGT test kms_flip.

[drm] *ERROR* PHY G Read 0d80 failed after 3 retries.
[drm] *ERROR* PHY G Write 0d81 failed after 3 retries.

Since the error is sporadic in nature, the patch proposes
to reset the message bus after every successful or unsuccessful
read or write operation. However, the testing revealed that this
alone is not sufficient method and therefore an additional
delay is introduced anything from 200us to 300us to let HW to
settle down. This delay is experimental value and has no
specification to back it up.

v2: Add FIXME's to indicate the experimental nature of
    this workaround (Rodrigo)

Signed-off-by: Mika Kahola <mika.kahola at intel.com>
---
 drivers/gpu/drm/i915/display/intel_cx0_phy.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/drivers/gpu/drm/i915/display/intel_cx0_phy.c b/drivers/gpu/drm/i915/display/intel_cx0_phy.c
index 6e6a1818071e..7c48ec5e54bd 100644
--- a/drivers/gpu/drm/i915/display/intel_cx0_phy.c
+++ b/drivers/gpu/drm/i915/display/intel_cx0_phy.c
@@ -221,6 +221,14 @@ static u8 __intel_cx0_read(struct drm_i915_private *i915, enum port port,
 	for (i = 0; i < 3; i++) {
 		status = __intel_cx0_read_once(i915, port, lane, addr);
 
+		/*
+		 * FIXME: Workaround to let HW to settle
+		 * down and let the message bus to end up
+		 * in a known state
+		 */
+		intel_cx0_bus_reset(i915, port, lane);
+		usleep_range(200, 300);
+
 		if (status >= 0)
 			return status;
 	}
@@ -300,6 +308,14 @@ static void __intel_cx0_write(struct drm_i915_private *i915, enum port port,
 	for (i = 0; i < 3; i++) {
 		status = __intel_cx0_write_once(i915, port, lane, addr, data, committed);
 
+		/*
+		 * FIXME: Workaround to let HW to settle
+		 * down and let the message bus to end up
+		 * in a known state
+		 */
+		intel_cx0_bus_reset(i915, port, lane);
+		usleep_range(200, 300);
+
 		if (status == 0)
 			return;
 	}
-- 
2.34.1