[PATCH 08/10] drm: Fix for DP CTS test 4.2.2.5 - I2C DEFER handling

[Paulo Zanoni]

2015-04-15 12:38 GMT-03:00 Todd Previte <tprevite at gmail.com>:
> 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>
(although just counting to 14 probably wouldn't hurt and would make
things simpler)

> ---
>  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 71dcbc6..7f0356e 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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-- 
Paulo Zanoni