[PATCH 7/8] drm/i915: Fix random aux transactions failures.

[Jani Nikula]

On Sat, 21 Nov 2015, Rodrigo Vivi <rodrigo.vivi at intel.com> wrote:
> Mainly aux communications on sink_crc
> were failing a lot randomly on recent platforms.
> The first solution was to try to use intel_dp_dpcd_read_wake, but then
> it was suggested to move retries to drm level.
>
> Since drm level was already taking care of retries and didn't want
> to through random retries on that level the second solution was to
> put the retries at aux_transfer layer what was nacked.
>
> So I realized we had so many retries in different places and
> started to organize that a bit. During this organization I noticed
> that we weren't handing at all the case were the message size was
> zeroed. And this was exactly the case that was affecting sink_crc.
>
> Also we weren't respect BSPec who says this size message = 0 or > 20
> are forbidden.
>
> It is a fact that we still have no clue why we are getting this
> forbidden value there. But anyway we need to handle that for now
> so we return -EBUSY and drm level takes care of the retries that
> are already in place.
>
> Cc: Jani Nikula <jani.nikula at intel.com>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> Signed-off-by: Rodrigo Vivi <rodrigo.vivi at intel.com>
> ---
>  drivers/gpu/drm/i915/intel_dp.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> index 35048d6..c87e937 100644
> --- a/drivers/gpu/drm/i915/intel_dp.c
> +++ b/drivers/gpu/drm/i915/intel_dp.c
> @@ -905,6 +905,17 @@ done:
>  	/* Unload any bytes sent back from the other side */
>  	recv_bytes = ((status & DP_AUX_CH_CTL_MESSAGE_SIZE_MASK) >>
>  		      DP_AUX_CH_CTL_MESSAGE_SIZE_SHIFT);
> +
> +	/*
> +	 * By BSpec: "Message sizes of 0 or >20 are not allowed."
> +	 * We have no idea of what happened so we return -EBUSY so
> +	 * drm layer takes care for the necessary retries.
> +	 */
> +	if (recv_bytes == 0 || recv_bytes > 20) {
> +		ret = -EBUSY;

This deserves debug logging at the very least.

BR,
Jani.


> +		goto out;
> +	}
> +
>  	if (recv_bytes > recv_size)
>  		recv_bytes = recv_size;

-- 
Jani Nikula, Intel Open Source Technology Center