Re: [PATCH 1/2] drm/i915: Do a dummy DPCD read before the actual read

[Todd Previte <tprevite@gmail.com>]

On 10/17/2014 1:43 AM, Jani Nikula wrote:
> On Thu, 16 Oct 2014, ville.syrjala@linux.intel.com wrote:
>> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
>>
>> Sometimes we seem to get utter garbage from DPCD reads. The resulting
>> buffer is filled with the same byte, and the operation completed without
>> errors. My HP ZR24w monitor seems particularly susceptible to this
>> problem once it's gone into a sleep mode.
>>
>> The issue seems to happen only for the first AUX message that wakes the
>> sink up. But as the first AUX read we often do is the DPCD receiver
>> cap it does wreak a bit of havoc with subsequent link training etc. when
>> the receiver cap bw/lane/etc. information is garbage.
> This makes me suspect our sink dpms and wake handling even more than I
> already did. Someone(tm) should dig into the DP and hw specs again with
> fresh eyes...
I can go look into this today/next week. This problem sounds vaguely 
similar to one I've run across before.

-T

>> A sufficient workaround seems to be to perform a single byte dummy read
>> before reading the actual data. I suppose that just wakes up the sink
>> sufficiently and we can just throw away the returned data in case it's
>> crap. DP_DPCD_REV seems like a sufficiently safe location to read here.
> Seems like a pretty harmless thing to do, and we already do loads of
> dpcd reads anyway. We should throw this at some related bugs.
>
> So ack.
>
> BR,
> Jani.
>
>
>> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
>> ---
>>   drivers/gpu/drm/i915/intel_dp.c | 7 +++++++
>>   1 file changed, 7 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
>> index 64c8e04..f07f02c 100644
>> --- a/drivers/gpu/drm/i915/intel_dp.c
>> +++ b/drivers/gpu/drm/i915/intel_dp.c
>> @@ -2870,6 +2870,13 @@ intel_dp_dpcd_read_wake(struct drm_dp_aux *aux, unsigned int offset,
>>   	ssize_t ret;
>>   	int i;
>>   
>> +	/*
>> +	 * Sometime we just get the same incorrect byte repeated
>> +	 * over the entire buffer. Doing just one throw away read
>> +	 * initially seems to "solve" it.
>> +	 */
>> +	drm_dp_dpcd_read(aux, DP_DPCD_REV, buffer, 1);
>> +
>>   	for (i = 0; i < 3; i++) {
>>   		ret = drm_dp_dpcd_read(aux, offset, buffer, size);
>>   		if (ret == size)
>> -- 
>> 2.0.4
>>
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