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

Paulo Zanoni
2015-04-16T20:23:30Z

2015-04-15 12:38 GMT-03:00 Todd Previte <tprevite@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@gmail.com>
> Cc: dri-devel@lists.freedesktop.org

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@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