[Intel-gfx] [PATCH 7/9] drm/i915: Fix for DP CTS test 4.2.2.5 - I2C DEFER handling

[Todd Previte]

On 4/6/15 5:05 PM, Paulo Zanoni wrote:
> 2015-03-31 14:15 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.
>>
>> Signed-off-by: Todd Previte <tprevite at gmail.com>
>> Cc: dri-devel at 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--;
> I wouldn't be surprised if someone discovers a monitor or some sort of
> dongle that keeps sending I2C defer errors forever, keeping us in an
> infinite loop. Shouldn't we count each error in separate? Or maybe
> just loop up to 14 times, in case that doesn't violate any spec (I
> didn't check)?
I think the safest thing to do would be to put a failsafe on the 
i2c_defer_counter. That would ensure that the compliance test gets its 7 
retries and that if we do encounter a misbehaving device, the driver 
won't let the unending defers cause an infinite loop. Updated patch 
shortly.

>>                          usleep_range(400, 500);
>>                          continue;
>>
>> --
>> 1.9.1
>>
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>
>