Re: [Intel-gfx] [PATCH] drm/i915/ilk-glk: Fix link training on links with LTTPRs

[Takashi Iwai <tiwai@suse.de>]

On Tue, 16 Mar 2021 17:54:26 +0100,
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).
> 
> 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.
> 
> Reported-by: Takashi Iwai <tiwai@suse.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>
> Signed-off-by: Imre Deak <imre.deak@intel.com>

Thanks!  I'm now building a test kernel and ask people testing it.
  https://apibugzilla.suse.com/show_bug.cgi?id=1183294

BTW, the patch isn't applicable cleanly to 5.11.x kernel as is.
The file intel_dp_aux.c needs to be changed to intel_dp.c.
Hopefully Greg can manage the file rename.


Takashi

> ---
>  drivers/gpu/drm/i915/display/intel_dp_aux.c           | 7 +++++++
>  drivers/gpu/drm/i915/display/intel_dp_link_training.c | 8 ++++++++
>  2 files changed, 15 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dp_aux.c b/drivers/gpu/drm/i915/display/intel_dp_aux.c
> index eaebf123310a..b581e8acce07 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 ILK-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..de6d70a29b47 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> @@ -123,10 +123,18 @@ intel_dp_set_lttpr_transparent_mode(struct intel_dp *intel_dp, bool enable)
>   */
>  int intel_dp_lttpr_init(struct intel_dp *intel_dp)
>  {
> +	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
>  	int lttpr_count;
>  	bool ret;
>  	int i;
>  
> +	/*
> +	 * 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 0;
> +
>  	if (intel_dp_is_edp(intel_dp))
>  		return 0;
>  
> -- 
> 2.25.1
> 
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