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

[Todd Previte <tprevite@gmail.com>]

On 10/17/2014 2:06 AM, Ville Syrjälä wrote:
> On Thu, Oct 16, 2014 at 12:39:29PM -0700, Todd Previte wrote:
>> On 10/16/2014 10:46 AM, 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.
>>>
>>> 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.
>>>
>>> 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)
>> Seems like a reasonable workaround for this problem, though
>> investigating the actual root cause might be worthwhile.
> Sure. If someone has an AUX analyzer and a HP ZR24w monitor it should
> be trivial to look at the traffic and see if there's something bogus in
> our AUX communication. Sadly I don't have an AUX analyzer.

I've got the monitor on my desk but no AUX analyzer to use. I'll see if 
I can track one down.

-T

>> Reviewed-by: Todd Previte <tprevite@gmail.com>
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