[Intel-gfx] [PATCH v4 2/4] drm/i915/psr: Account for sink CRC raciness on some panels

[Rodrigo Vivi]

On Tue, Jul 18, 2017 at 2:34 PM, Jim Bride <jim.bride at linux.intel.com> wrote:
> According to the eDP spec, when the count field in TEST_SINK_MISC
> increments then the six bytes of sink CRC information in the DPCD
> should be valid.  Unfortunately, this doesn't seem to be the case
> on some panels, and as a result we get some incorrect and inconsistent
> values from the sink CRC DPCD locations at times.  This problem exhibits
> itself more on faster processors (relative failure rates HSW < SKL < KBL.)
> In order to try and account for this, we try a lot harder to read the sink
> CRC until we get consistent values twice in a row before returning what we
> read and delay for a time before trying to read.  We still see some
> occasional failures, but reading the sink CRC is much more reliable,
> particularly on SKL and KBL, with these changes than without.

Is DK now ok with this description?
I believe he requested more info here.

>
> v2: * Reduce number of retries when reading the sink CRC (Jani)
>     * Refactor to minimize changes to the code (Jani)
>     * Rebase
> v3: * Rebase
> v4: * Switch from do-while to for loop when reading CRC values (Jani)
>     * Rebase
> Cc: Rodrigo Vivi <rodrigo.vivi at intel.com>
> Cc: Paulo Zanoni <paulo.r.zanoni at intel.com>
> Cc: Jani Nikula <jani.nikula at intel.com>
> Signed-off-by: Jim Bride <jim.bride at linux.intel.com>
> ---
>  drivers/gpu/drm/i915/intel_dp.c | 33 ++++++++++++++++++++++++++++++---
>  1 file changed, 30 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> index 2d42d09..c90ca1c 100644
> --- a/drivers/gpu/drm/i915/intel_dp.c
> +++ b/drivers/gpu/drm/i915/intel_dp.c
> @@ -3906,6 +3906,11 @@ int intel_dp_sink_crc(struct intel_dp *intel_dp, u8 *crc)
>         u8 buf;
>         int count, ret;
>         int attempts = 6;
> +       u8 old_crc[6];
> +
> +       if (crc == NULL) {
> +               return -ENOMEM;
> +       }

wouldn't we drop this check per DK and Jani request?
I believe we don't need it, but even if there are cases that we need
we could remove the braces..

>
>         ret = intel_dp_sink_crc_start(intel_dp);
>         if (ret)
> @@ -3929,11 +3934,33 @@ int intel_dp_sink_crc(struct intel_dp *intel_dp, u8 *crc)
>                 goto stop;
>         }
>
> -       if (drm_dp_dpcd_read(&intel_dp->aux, DP_TEST_CRC_R_CR, crc, 6) < 0) {
> -               ret = -EIO;
> -               goto stop;
> +       /*
> +        * Sometimes it takes a while for the "real" CRC values to land in
> +        * the DPCD, so try several times until we get two reads in a row
> +        * that are the same.  If we're an eDP panel, delay between reads
> +        * for a while since the values take a bit longer to propagate.
> +        */
> +       for (attempts = 0; attempts < 6; attempts++) {
> +               intel_wait_for_vblank(dev_priv, intel_crtc->pipe);

DK, we need vblank wait because the crc calculation also may take one vblank.
usually 2 actually... one to make sure you have the full screen
updated and one for the calculation.
In the past when we didn't used the count we were waiting 2 vblanks...

> +
> +               if (drm_dp_dpcd_read(&intel_dp->aux, DP_TEST_CRC_R_CR,
> +                                    crc, 6) < 0) {
> +                       ret = -EIO;
> +                       break;
> +               }
> +
> +               if (attempts && memcmp(old_crc, crc, 6) == 0)
> +                       break;
> +               memcpy(old_crc, crc, 6);

little bikeshed: too many hardcoded "6" around... a sizeof would be better...
but whatever...

> +
> +               if (is_edp(intel_dp))
> +                       usleep_range(20000, 25000);
>         }
>
> +       if (attempts == 6) {
> +               DRM_DEBUG_KMS("Failed to get CRC after 6 attempts.\n");
> +               ret = -ETIMEDOUT;
> +       }
>  stop:
>         intel_dp_sink_crc_stop(intel_dp);
>         return ret;
> --
> 2.7.4
>
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-- 
Rodrigo Vivi
Blog: http://blog.vivi.eng.br