Re: [PATCH v4 4/4] drm/msm/dp: Add support for LTTPR handling

[Abhinav Kumar <quic_abhinavk@quicinc.com>]

On 1/8/2025 6:31 AM, 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.
> 
> According to the DisplayPort standard, LTTPRs have two operating
> modes:
>   - non-transparent - it replies to DPCD LTTPR field specific AUX
>     requests, while passes through all other AUX requests
>   - transparent - it passes through all AUX requests.
> 
> Switching between this two modes is done by the DPTX by issuing
> an AUX write to the DPCD PHY_REPEATER_MODE register.
> 
> 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.
> 
> 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.
> 
> Tested-by: Johan Hovold <johan+linaro@kernel.org>
> Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
> Reviewed-by: Johan Hovold <johan+linaro@kernel.org>
> Signed-off-by: Abel Vesa <abel.vesa@linaro.org>
> ---
>   drivers/gpu/drm/msm/dp/dp_display.c | 17 +++++++++++++++++
>   1 file changed, 17 insertions(+)
> 
> diff --git a/drivers/gpu/drm/msm/dp/dp_display.c b/drivers/gpu/drm/msm/dp/dp_display.c
> index 24dd37f1682bf5016bb0efbeb44489061deff060..ad09daa4c8ab5c0eb67890509b94e72820bab870 100644
> --- a/drivers/gpu/drm/msm/dp/dp_display.c
> +++ b/drivers/gpu/drm/msm/dp/dp_display.c
> @@ -107,6 +107,8 @@ struct msm_dp_display_private {
>   	struct msm_dp_event event_list[DP_EVENT_Q_MAX];
>   	spinlock_t event_lock;
>   
> +	u8 lttpr_caps[DP_LTTPR_COMMON_CAP_SIZE];
> +

The reason downstream stored it panel is to read it first in dp_panel's 
read_sink_caps and call lttpr_init if drm_dp_lttpr_count() is non-zero.

But here it looks like  msm_dp_display_lttpr_init() internally handles 
this for us. So no need to store this?

>   	bool wide_bus_supported;
>   
>   	struct msm_dp_audio *audio;
> @@ -367,12 +369,27 @@ static int msm_dp_display_send_hpd_notification(struct msm_dp_display_private *d
>   	return 0;
>   }
>   
> +static void msm_dp_display_lttpr_init(struct msm_dp_display_private *dp)
> +{
> +	int rc;
> +
> +	if (drm_dp_read_lttpr_common_caps(dp->aux, dp->panel->dpcd,
> +					  dp->lttpr_caps))
> +		return;
> +
> +	rc = drm_dp_lttpr_init(dp->aux, drm_dp_lttpr_count(dp->lttpr_caps));
> +	if (rc)
> +		DRM_ERROR("failed to set LTTPRs transparency mode, rc=%d\n", rc);
> +}
> +
>   static int msm_dp_display_process_hpd_high(struct msm_dp_display_private *dp)
>   {
>   	struct drm_connector *connector = dp->msm_dp_display.connector;
>   	const struct drm_display_info *info = &connector->display_info;
>   	int rc = 0;
>   
> +	msm_dp_display_lttpr_init(dp);
> +

Can you pls move this call after msm_dp_panel_read_sink_caps()?

If msm_dp_panel_read_sink_caps() fails there is no need to call 
msm_dp_display_lttpr_init().


>   	rc = msm_dp_panel_read_sink_caps(dp->panel, connector);
>   	if (rc)
>   		goto end;
>