Re: [PATCH 3/9] drm/i915: Add a delay in Displayport AUX transactions for compliance testing

[Todd Previte <tprevite@gmail.com>]

On 4/1/2015 11:23 AM, Ville Syrjälä wrote:
> On Tue, Mar 31, 2015 at 10:15:00AM -0700, Todd Previte wrote:
>> The Displayport Link Layer Compliance Testing Specification 1.2 rev 1.1
>> specifies that repeated AUX transactions after a failure (no response /
>> invalid response) must have a minimum delay of 400us before the resend can
>> occur. Tests 4.2.1.1 and 4.2.1.2 are two tests that require this specifically.
>>
>> V2:
>> - Changed udelay() to usleep_range()
>> V3:
>> - Removed extraneous check for timeout
>> - Updated comment to reflect this change
>>
>> Signed-off-by: Todd Previte <tprevite@gmail.com>
>> ---
>>   drivers/gpu/drm/i915/intel_dp.c | 10 ++++++++--
>>   1 file changed, 8 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
>> index ed2f60c..dc87276 100644
>> --- a/drivers/gpu/drm/i915/intel_dp.c
>> +++ b/drivers/gpu/drm/i915/intel_dp.c
>> @@ -877,9 +877,15 @@ intel_dp_aux_ch(struct intel_dp *intel_dp,
>>   				   DP_AUX_CH_CTL_TIME_OUT_ERROR |
>>   				   DP_AUX_CH_CTL_RECEIVE_ERROR);
>>   
>> -			if (status & (DP_AUX_CH_CTL_TIME_OUT_ERROR |
>> -				      DP_AUX_CH_CTL_RECEIVE_ERROR))
>> +			/* DP CTS 1.2 Core Rev 1.1, 4.2.1.1 & 4.2.1.2
>> +			     400us delay required for errors and timeouts
>> +			     Timeout errors from the HW already meet this
>> +			     requirement so skip to next iteration
>> +			*/
> Weird format for multi line comment.
Yeah I had to squish it in there to keep it under 80 columns. Needs the 
'*' on the left side too though. I'll fix that and repost.
>
>> +			if (status & DP_AUX_CH_CTL_RECEIVE_ERROR) {
>> +				usleep_range(400, 500);
>>   				continue;
>> +			}
> Where did the DP_AUX_CH_CTL_TIME_OUT_ERROR handling go?
As I recall, previous review feedback indicated that the timeout 
condition there was already accounted for.

on 12/15, Paulo commented:

One thing to notice is that if we get a TIME_OUT_ERROR I guess it
means we already waited our standard timeout (which is either 400, 600
or 1600, depending on the platform), so shouldn't we just do the
usleep() after the RECEIVE_ERROR error?


When I checked the BSpec, that seemed to be the case so I removed the 
TIME_OUT_ERROR. Without this
code in place, we still pass the compliance tests for AUX transactions, 
one of which is for a no-reply transaction.
That case specifically should hit the TIME_OUT_ERROR if it was going to 
occur, I would think.

If you can give me a case where that becomes an issue, it's a simple fix 
to add it back in there.

>
>>   			if (status & DP_AUX_CH_CTL_DONE)
>>   				break;
>>   		}
>> -- 
>> 1.9.1
>>
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