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

Rodrigo Vivi
2017-08-03T18:07:48Z

On Tue, Jul 18, 2017 at 2:34 PM, Jim Bride <jim.bride@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@intel.com>
> Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
> Cc: Jani Nikula <jani.nikula@intel.com>
> Signed-off-by: Jim Bride <jim.bride@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