[PATCH RFC 4/4] drm/msm/dp: Add support for LTTPR handling

[Dmitry Baryshkov]

On Thu, Oct 31, 2024 at 05:12:48PM +0200, Abel Vesa wrote:
> Link Training Tunable PHY Repeaters (LTTPRs) are defined in DisplayPort
> 1.4a specification. As the name suggests, these PHY repeaters are
> capable of adjusting their output for link training purposes.
> 
> The msm DP driver is currently lacking any handling of LTTPRs.
> This means that if at least one LTTPR is found between DPTX and DPRX,
> the link training would fail if that LTTPR was not already configured
> in transparent mode.

It might be nice to mention what is the transparent mode, especially for
those who do not have the standard at hand.

> The section 3.6.6.1 from the DisplayPort v2.0 specification mandates
> that before link training with the LTTPR is started, the DPTX may place
> the LTTPR in non-transparent mode by first switching to transparent mode
> and then to non-transparent mode. This operation seems to be needed only
> on first link training and doesn't need to be done again until device is
> unplugged.
> 
> It has been observed on a few X Elite-based platforms which have
> such LTTPRs in their board design that the DPTX needs to follow the
> procedure described above in order for the link training to be successful.
> 
> So add support for reading the LTTPR DPCD caps to figure out the number
> of such LTTPRs first. Then, for platforms (or Type-C dongles) that have
> at least one such an LTTPR, set its operation mode to transparent mode
> first and then to non-transparent, just like the mentioned section of
> the specification mandates.
> 
> Signed-off-by: Abel Vesa <abel.vesa at linaro.org>
> ---
>  drivers/gpu/drm/msm/dp/dp_display.c | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/drivers/gpu/drm/msm/dp/dp_display.c b/drivers/gpu/drm/msm/dp/dp_display.c
> index f01980b0888a40b719d3958cb96c6341feada077..5d3d318d7b87ce3bf567d8b7435931d8e087f713 100644
> --- a/drivers/gpu/drm/msm/dp/dp_display.c
> +++ b/drivers/gpu/drm/msm/dp/dp_display.c
> @@ -107,6 +107,8 @@ struct dp_display_private {
>  	struct dp_event event_list[DP_EVENT_Q_MAX];
>  	spinlock_t event_lock;
>  
> +	u8 lttpr_caps[DP_LTTPR_COMMON_CAP_SIZE];
> +
>  	bool wide_bus_supported;
>  
>  	struct dp_audio *audio;
> @@ -367,12 +369,35 @@ static int dp_display_send_hpd_notification(struct dp_display_private *dp,
>  	return 0;
>  }
>  
> +static void dp_display_lttpr_init(struct dp_display_private *dp)
> +{
> +	int lttpr_count;
> +
> +	if (drm_dp_read_lttpr_common_caps(dp->aux, dp->panel->dpcd,
> +					  dp->lttpr_caps))
> +		return;
> +
> +	lttpr_count = drm_dp_lttpr_count(dp->lttpr_caps);
> +
> +	if (lttpr_count) {
> +		drm_dp_lttpr_set_transparent_mode(dp->aux, true);
> +
> +		if (lttpr_count > 0) {
> +			if (drm_dp_lttpr_set_transparent_mode(dp->aux, false) != 1)
> +				drm_dp_lttpr_set_transparent_mode(dp->aux, true);
> +		}
> +	}
> +}
> +
>  static int dp_display_process_hpd_high(struct dp_display_private *dp)
>  {
>  	struct drm_connector *connector = dp->dp_display.connector;
>  	const struct drm_display_info *info = &connector->display_info;
>  	int rc = 0;
>  
> +	if (!dp->dp_display.is_edp)
> +		dp_display_lttpr_init(dp);

Why is it limited to non-eDP cases only.

> +
>  	rc = dp_panel_read_sink_caps(dp->panel, connector);
>  	if (rc)
>  		goto end;
> 
> -- 
> 2.34.1
> 

-- 
With best wishes
Dmitry