Re: [PATCH v2 1/3] drm/i915/ilk-glk: Fix link training on links with LTTPRs

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Wed, Mar 17, 2021 at 08:48:59PM +0200, Imre Deak wrote:
> The spec requires to use at least 3.2ms for the AUX timeout period if
> there are LT-tunable PHY Repeaters on the link (2.11.2). An upcoming
> spec update makes this more specific, by requiring a 3.2ms minimum
> timeout period for the LTTPR detection reading the 0xF0000-0xF0007
> range (3.6.5.1).

I'm pondering if we could reduce the timeout after having determined
wherther LTTPRs are present or not? But maybe that wouldn't really speed
up anything since we can't reduce the timeout until after detecting
*something*. And once there is something there we shouldn't really get
any more timeouts I guess. So probably a totally stupid idea.

Anyways, this seems about the only thing we can do given the limited
hw capabilities.
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

> 
> Accordingly disable LTTPR detection until GLK, where the maximum timeout
> we can set is only 1.6ms.
> 
> Link training in the non-transparent mode is known to fail at least on
> some SKL systems with a WD19 dock on the link, which exposes an LTTPR
> (see the References below). While this could have different reasons
> besides the too short AUX timeout used, not detecting LTTPRs (and so not
> using the non-transparent LT mode) fixes link training on these systems.
> 
> While at it add a code comment about the platform specific maximum
> timeout values.
> 
> v2: Add a comment about the g4x maximum timeout as well. (Ville)
> 
> Reported-by: Takashi Iwai <tiwai@suse.de>
> Reported-and-tested-by: Santiago Zarate <santiago.zarate@suse.com>
> Reported-and-tested-by: Bodo Graumann <mail@bodograumann.de>
> References: https://gitlab.freedesktop.org/drm/intel/-/issues/3166
> Fixes: b30edfd8d0b4 ("drm/i915: Switch to LTTPR non-transparent mode link training")
> Cc: <stable@vger.kernel.org> # v5.11
> Cc: Takashi Iwai <tiwai@suse.de>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Signed-off-by: Imre Deak <imre.deak@intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_dp_aux.c       |  7 +++++++
>  .../gpu/drm/i915/display/intel_dp_link_training.c | 15 ++++++++++++---
>  2 files changed, 19 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dp_aux.c b/drivers/gpu/drm/i915/display/intel_dp_aux.c
> index eaebf123310a..10fe17b7280d 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_aux.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_aux.c
> @@ -133,6 +133,7 @@ static u32 g4x_get_aux_send_ctl(struct intel_dp *intel_dp,
>  	else
>  		precharge = 5;
>  
> +	/* Max timeout value on G4x-BDW: 1.6ms */
>  	if (IS_BROADWELL(dev_priv))
>  		timeout = DP_AUX_CH_CTL_TIME_OUT_600us;
>  	else
> @@ -159,6 +160,12 @@ static u32 skl_get_aux_send_ctl(struct intel_dp *intel_dp,
>  	enum phy phy = intel_port_to_phy(i915, dig_port->base.port);
>  	u32 ret;
>  
> +	/*
> +	 * Max timeout values:
> +	 * SKL-GLK: 1.6ms
> +	 * CNL: 3.2ms
> +	 * ICL+: 4ms
> +	 */
>  	ret = DP_AUX_CH_CTL_SEND_BUSY |
>  	      DP_AUX_CH_CTL_DONE |
>  	      DP_AUX_CH_CTL_INTERRUPT |
> 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 19ba7c7cbaab..c0e25c75c105 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> @@ -82,6 +82,18 @@ static void intel_dp_read_lttpr_phy_caps(struct intel_dp *intel_dp,
>  
>  static bool intel_dp_read_lttpr_common_caps(struct intel_dp *intel_dp)
>  {
> +	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
> +
> +	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 (INTEL_GEN(i915) < 10)
> +		return false;
> +
>  	if (drm_dp_read_lttpr_common_caps(&intel_dp->aux,
>  					  intel_dp->lttpr_common_caps) < 0) {
>  		memset(intel_dp->lttpr_common_caps, 0,
> @@ -127,9 +139,6 @@ int intel_dp_lttpr_init(struct intel_dp *intel_dp)
>  	bool ret;
>  	int i;
>  
> -	if (intel_dp_is_edp(intel_dp))
> -		return 0;
> -
>  	ret = intel_dp_read_lttpr_common_caps(intel_dp);
>  	if (!ret)
>  		return 0;
> -- 
> 2.25.1

-- 
Ville Syrjälä
Intel