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

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

On Tue, Mar 16, 2021 at 06:54:26PM +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).
> 
> 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>
> ---
>  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 */

also g4x

>  	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;

I don't understand how detecting the LTTPR affects this? The LTTPRs will
still snoop stuff and do their hidden magic even if we don't know
they're there. How does leaving them in transparent mode speed up
whatever they do?

Also, maybe we should just bump the timeout to the max on all platforms?

> +
>  	if (intel_dp_is_edp(intel_dp))
>  		return 0;
>  
> -- 
> 2.25.1
> 
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-- 
Ville Syrjälä
Intel
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