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.v...@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
