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

[Todd Previte]

On 4/7/15 7:29 AM, Paulo Zanoni wrote:
> 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.
To capture the discussion from IRC:

The primary issue previously was the potential for an infinite loop when 
decrementing the retry count. That is clearly addressed by this code, by 
only decrementing the loop while the defer count is below 8. In 
actuality, that needs to be 7, so a followup patch with that change will 
be posted shortly. There are other solutions (Ville has one in his 
reply), but this one works correctly, is minimally invasive and doesn't 
require changes to the loop structure.

The defer counter isn't used anywhere outside of Displayport compliance 
testing. In fact, the counters in the aux struct didn't even exist until 
I put them in there in a previous patch to support this exact test case. 
So there really isn't a non-DP compliance test specific solution to be 
had here. It's also unlikely that this has any significant effect on 
normal operations. In the event that a device misbehaves and begins 
issuing loads of defers, this code would only delay it by at most 7 
retries before the counter exceeded its value.

Since this value is not used outside of compliance testing, a reset 
per-transaction is unnecessary. Additionally, that would break the EDID 
compliance testing code as it would have no way of detecting that the 
read had failed due to continuous defers. As long as the counter is 
reset for each test request (which it is), this code will function 
properly for both real world and compliance operations.

As indicated in the comments, this code exists to ensure compliance with 
test 4.2.2.5 from the Displayport Link CTS Core 1.2 rev1.1. This test 
verifies that the source device is capable of responding to a failure to 
read the EDID when the sink device continuously defers the transaction. 
It ensures that at least 7 I2C defers are received before calling it 
quits, versus simply retrying the transaction 7 times regardless of the 
failure mode.

Since I'm going to post an updated patch anyways, I will likely roll the 
defer count increment into the if-statement to remove one line of code. 
That also requires moving the increment to a pre- versus post- 
condition, as follows:

     ...
     if (++aux->i2c_defer_count < 7)
         ...
     ...

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

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