[PATCH 07/11] drm/i915: Fix for DP CTS test 4.2.2.5 - I2C DEFER handling

[Paulo Zanoni]

2015-04-06 23:11 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 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.
>
> 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.
>
> Signed-off-by: Todd Previte <tprevite at gmail.com>
> Cc: dri-devel at lists.freedesktop.org
> ---
>  drivers/gpu/drm/drm_dp_helper.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
> index 79968e3..23025cf 100644
> --- a/drivers/gpu/drm/drm_dp_helper.c
> +++ b/drivers/gpu/drm/drm_dp_helper.c
> @@ -469,6 +469,12 @@ 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
> +                        */
> +                       if (aux->i2c_defer_count < 8)

I don't think this is the way to go. During normal
(non-compliance-testing) operation we never zero i2c_defer_count, so
we can't expect this to work, since we may start drm_dp_i2c_do_msg
with a i2c_defer_count value different than zero. Also, during i915.ko
DP compliance we only zero i2c_defer_count at the very beginning of
each test, not at every aux transaction, and I really think we need a
solution that is not specific to compliance testing.


> +                               retry--;
>                         usleep_range(400, 500);
>                         continue;
>
> --
> 1.9.1
>
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-- 
Paulo Zanoni