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

[Jani Nikula <jani.nikula@linux.intel.com>]

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...

> 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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-- 
Jani Nikula, Intel Open Source Technology Center
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