[PATCH 4/5] drm: Fix for DP CTS test 4.2.2.5 - I2C DEFER handling

[Daniel Vetter]

On Sat, Apr 18, 2015 at 12:04:18AM -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 add the numer of defers to 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.
> 
> V2:
> - Added a check on the number of I2C Defers to limit the number
>   of times that the retries variable will be decremented. This
>   is to address review feedback regarding possible infinite loops
>   from misbehaving sink devices.
> V3:
> - Fixed the limit value to 7 instead of 8 to get the correct retry
>   count.
> - Combined the increment of the defer count into the if-statement
> V4:
> - Removed i915 tag from subject as the patch is not i915-specific
> V5:
> - Updated the for-loop to add the number of i2c defers to the retry
>   counter such that the correct number of retry attempts will be
>   made
> 
> Signed-off-by: Todd Previte <tprevite at gmail.com>
> Cc: dri-devel at lists.freedesktop.org
> Reviewed-by: Paulo Zanoni <paulo.r.zanoni at intel.com>

Applied to topic/drm-misc.
-Daniel

> ---
>  drivers/gpu/drm/drm_dp_helper.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
> index 575172e..80a02a4 100644
> --- a/drivers/gpu/drm/drm_dp_helper.c
> +++ b/drivers/gpu/drm/drm_dp_helper.c
> @@ -432,7 +432,7 @@ static u32 drm_dp_i2c_functionality(struct i2c_adapter *adapter)
>   */
>  static int drm_dp_i2c_do_msg(struct drm_dp_aux *aux, struct drm_dp_aux_msg *msg)
>  {
> -	unsigned int retry;
> +	unsigned int retry, defer_i2c;
>  	int ret;
>  
>  	/*
> @@ -440,7 +440,7 @@ static int drm_dp_i2c_do_msg(struct drm_dp_aux *aux, struct drm_dp_aux_msg *msg)
>  	 * is required to retry at least seven times upon receiving AUX_DEFER
>  	 * before giving up the AUX transaction.
>  	 */
> -	for (retry = 0; retry < 7; retry++) {
> +	for (retry = 0, defer_i2c = 0; retry < (7 + defer_i2c); retry++) {
>  		mutex_lock(&aux->hw_mutex);
>  		ret = aux->transfer(aux, msg);
>  		mutex_unlock(&aux->hw_mutex);
> @@ -499,7 +499,13 @@ 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");
> +			/* DP Compliance Test 4.2.2.5 Requirement:
> +			 * Must have at least 7 retries for I2C defers on the
> +			 * transaction to pass this test
> +			 */
>  			aux->i2c_defer_count++;
> +			if (defer_i2c < 7)
> +				defer_i2c++;
>  			usleep_range(400, 500);
>  			continue;
>  
> -- 
> 1.9.1
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch