[PATCH 1/5] drm/i915: Fix for DP CTS test 4.2.2.5 - I2C DEFER handling

[PATCH 1/5] drm/i915: Fix for DP CTS test 4.2.2.5 - I2C DEFER handling

[Todd Previte]

For test 4.2.2.5 to pass per the Link CTS Core 1.2 rev1.1 spec, the source
device must attempt at least 7 times to read the EDID when it receives an
I2C defer. The normal DRM code makes only 7 retries, regardless of whether
or not the response is a native defer or an I2C defer. Test 4.2.2.5 fails
since there are native defers interspersed with the I2C defers which
results in less than 7 EDID read attempts.

The solution is to decrement the retry counter when an I2C DEFER is returned
such that another read attempt will be made. This situation should normally
only occur in compliance testing, however, as a worse case real-world
scenario, it would result in 13 attempts ( 6 native defers, 7 I2C defers)
for a single transaction to complete. The net result is a slightly slower
response to an EDID read that shouldn't significantly impact overall
performance.

Signed-off-by: Todd Previte <tprevite@gmail.com>
Cc: dri-devel@lists.freedesktop.org
---
 drivers/gpu/drm/drm_dp_helper.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index 79968e3..0539758 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -469,6 +469,11 @@ static int drm_dp_i2c_do_msg(struct drm_dp_aux *aux, struct drm_dp_aux_msg *msg)
 		case DP_AUX_I2C_REPLY_DEFER:
 			DRM_DEBUG_KMS("I2C defer\n");
 			aux->i2c_defer_count++;
+			/* DP Compliance Test 4.2.2.5 Requirement:
+			 * Must have at least 7 retries for I2C defers on the
+			 * transaction to pass this test
+			 */
+			retry--;
 			usleep_range(400, 500);
 			continue;
 
-- 
1.9.1

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Re: [PATCH 1/5] drm/i915: Fix for DP CTS test 4.2.2.5 - I2C DEFER handling

[Ville Syrjälä]

On Wed, Apr 01, 2015 at 10:52:58AM -0700, Todd Previte wrote:
> For test 4.2.2.5 to pass per the Link CTS Core 1.2 rev1.1 spec, the source
> device must attempt at least 7 times to read the EDID when it receives an
> I2C defer. The normal DRM code makes only 7 retries, regardless of whether
> or not the response is a native defer or an I2C defer. Test 4.2.2.5 fails
> since there are native defers interspersed with the I2C defers which
> results in less than 7 EDID read attempts.
> 
> The solution is to decrement the retry counter when an I2C DEFER is returned
> such that another read attempt will be made. This situation should normally
> only occur in compliance testing, however, as a worse case real-world
> scenario, it would result in 13 attempts ( 6 native defers, 7 I2C defers)
> for a single transaction to complete. The net result is a slightly slower
> response to an EDID read that shouldn't significantly impact overall
> performance.
> 
> Signed-off-by: Todd Previte <tprevite@gmail.com>
> Cc: dri-devel@lists.freedesktop.org
> ---
>  drivers/gpu/drm/drm_dp_helper.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
> index 79968e3..0539758 100644
> --- a/drivers/gpu/drm/drm_dp_helper.c
> +++ b/drivers/gpu/drm/drm_dp_helper.c
> @@ -469,6 +469,11 @@ static int drm_dp_i2c_do_msg(struct drm_dp_aux *aux, struct drm_dp_aux_msg *msg)
>  		case DP_AUX_I2C_REPLY_DEFER:
>  			DRM_DEBUG_KMS("I2C defer\n");
>  			aux->i2c_defer_count++;
> +			/* DP Compliance Test 4.2.2.5 Requirement:
> +			 * Must have at least 7 retries for I2C defers on the
> +			 * transaction to pass this test
> +			 */
> +			retry--;

That could lead to an infinite loop. I think what we need to do is
count the native and i2c defers separately, and abort if either
exceeds the limit.

>  			usleep_range(400, 500);
>  			continue;
>  
> -- 
> 1.9.1
> 
> _______________________________________________
> dri-devel mailing list
> dri-devel@lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/dri-devel

-- 
Ville Syrjälä
Intel OTC
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