Re: [Intel-gfx] [PATCH v3 2/2] drm/i915/dp: Add workaround for spurious AUX timeouts/hotplugs on LTTPR links

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

On Sat, 09 Apr 2022, Imre Deak <imre.deak@intel.com> wrote:
> Some ADLP DP link configuration at least with multiple LTTPRs expects
> the first DPCD access during the LTTPR/DPCD detection after hotplug to
> be a read from the LTTPR range starting with
> DP_LT_TUNABLE_PHY_REPEATER_FIELD_DATA_STRUCTURE_REV. The side effect of
> this read is to put each LTTPR into the LTTPR transparent or LTTPR
> non-transparent mode.
>
> The lack of the above read may leave some of the LTTPRs in non-LTTPR
> mode, while other LTTPRs in LTTPR transparent or LTTPR non-transparent
> mode (for instance LTTPRs after system suspend/resume that kept their
> mode from before suspend). Due to the different AUX timeouts the
> different modes imply, the DPCD access from a non-LTTPR range will
> timeout and lead to an LTTPR generated hotplug towards the source (which
> the LTTPR firmware uses to account for buggy TypeC adapters with a long
> wake-up delay).
>
> To avoid the above AUX timeout and subsequent hotplug interrupt, make
> sure that the first DPCD access during detection is a read from an
> LTTPR register.
>
> VLK: SYSCROS-72939
>
> v2: Keep DPCD read-out working on non-LTTPR platforms.
>
> Signed-off-by: Imre Deak <imre.deak@intel.com>

I kinda dislike how complicated this looks for a diff so small. *shrug*.
Seems to do what it says on the box,

Reviewed-by: Jani Nikula <jani.nikula@intel.com>

> ---
>  .../drm/i915/display/intel_dp_link_training.c | 33 ++++++++++---------
>  1 file changed, 17 insertions(+), 16 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_dp_link_training.c b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> index 26f9e2b748e40..9feaf1a589f38 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> @@ -82,19 +82,8 @@ static bool intel_dp_read_lttpr_common_caps(struct intel_dp *intel_dp,
>  					    const u8 dpcd[DP_RECEIVER_CAP_SIZE])
>  {
>  	struct intel_encoder *encoder = &dp_to_dig_port(intel_dp)->base;
> -	struct drm_i915_private *i915 = to_i915(encoder->base.dev);
>  	int ret;
>  
> -	if (intel_dp_is_edp(intel_dp))
> -		return false;
> -
> -	/*
> -	 * Detecting LTTPRs must be avoided on platforms with an AUX timeout
> -	 * period < 3.2ms. (see DP Standard v2.0, 2.11.2, 3.6.6.1).
> -	 */
> -	if (DISPLAY_VER(i915) < 10 || IS_GEMINILAKE(i915))
> -		return false;
> -
>  	ret = drm_dp_read_lttpr_common_caps(&intel_dp->aux, dpcd,
>  					    intel_dp->lttpr_common_caps);
>  	if (ret < 0)
> @@ -197,13 +186,25 @@ static int intel_dp_init_lttpr(struct intel_dp *intel_dp, const u8 dpcd[DP_RECEI
>   */
>  int intel_dp_init_lttpr_and_dprx_caps(struct intel_dp *intel_dp)
>  {
> -	u8 dpcd[DP_RECEIVER_CAP_SIZE];
> -	int lttpr_count;
> +	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
> +	int lttpr_count = 0;
>  
> -	if (drm_dp_read_dpcd_caps(&intel_dp->aux, dpcd))
> -		return -EIO;
> +	/*
> +	 * Detecting LTTPRs must be avoided on platforms with an AUX timeout
> +	 * period < 3.2ms. (see DP Standard v2.0, 2.11.2, 3.6.6.1).
> +	 */
> +	if (!intel_dp_is_edp(intel_dp) &&
> +	    (DISPLAY_VER(i915) >= 10 && !IS_GEMINILAKE(i915))) {
> +		u8 dpcd[DP_RECEIVER_CAP_SIZE];
>  
> -	lttpr_count = intel_dp_init_lttpr(intel_dp, dpcd);
> +		if (drm_dp_dpcd_probe(&intel_dp->aux, DP_LT_TUNABLE_PHY_REPEATER_FIELD_DATA_STRUCTURE_REV))
> +			return -EIO;
> +
> +		if (drm_dp_read_dpcd_caps(&intel_dp->aux, dpcd))
> +			return -EIO;
> +
> +		lttpr_count = intel_dp_init_lttpr(intel_dp, dpcd);
> +	}
>  
>  	/*
>  	 * The DPTX shall read the DPRX caps after LTTPR detection, so re-read

-- 
Jani Nikula, Intel Open Source Graphics Center