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

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

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
> 
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-- 
Ville Syrjälä
Intel OTC
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