Re: [PATCH v3 2/4] drm/i915/psr: Account for sink CRC raciness on some panels

Dhinakaran Pandiyan
2017-07-12T09:42:43Z

On Tuesday, July 11, 2017 3:19:54 PM PDT Jim Bride 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.

What's the goal here? Is this retry loop being added to deal with an IGT
failure? A bit of context as to how this is related to PSR will be helpful.

>
> v2: * Reduce number of retries when reading the sink CRC (Jani)
> * Refactor to minimize changes to the code (Jani)
> * Rebase
>
> Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
> Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
> Cc: Jani Nikula <jani.nikula@intel.com>
> Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
> Signed-off-by: Jim Bride <jim.bride@linux.intel.com>
> ---
> drivers/gpu/drm/i915/intel_dp.c | 40
> ++++++++++++++++++++++++++++++++++++---- 1 file changed, 36 insertions(+),
> 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_dp.c
> b/drivers/gpu/drm/i915/intel_dp.c index 2d42d09..69c8130c 100644
> --- a/drivers/gpu/drm/i915/intel_dp.c
> +++ b/drivers/gpu/drm/i915/intel_dp.c
> @@ -3906,6 +3906,14 @@ 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) {

Why is this needed?

> + memset(crc, 0, 6);
> + memset(old_crc, 0xff, 6);

I don't know much about crc values, is 0xff a known invalid value?

> + } else {
> + return -ENOMEM;
> + }
>
> ret = intel_dp_sink_crc_start(intel_dp);
> if (ret)
> @@ -3929,11 +3937,35 @@ 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;
> - }
> + attempts = 6;
> +
> + /*
> + * 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.
> + */
> + do {
> + intel_wait_for_vblank(dev_priv, intel_crtc->pipe);

Why wait for a vblank if the idea is to check for consistent crc values for
the same frame (I'm assuming that is the idea) ?

> + if (is_edp(intel_dp))
> + usleep_range(20000, 25000);
> +
> + if (drm_dp_dpcd_read(&intel_dp->aux, DP_TEST_CRC_R_CR,
> + crc, 6) < 0) {
> + ret = -EIO;
> + goto stop;
> + }
> +
> + if (memcmp(old_crc, crc, 6) == 0) {
> + ret = 0;
> + goto stop;
> + } else {
> + memcpy(old_crc, crc, 6);
> + }
> + } while (--attempts);
>
> + DRM_DEBUG_KMS("Failed to get CRC after 6 attempts.\n");
> + ret = -ETIMEDOUT;
> stop:
> intel_dp_sink_crc_stop(intel_dp);
> return ret;