On Tue, Dec 03, 2024 at 04:13:22PM +0800, Xiangxu Yin wrote:
>
>
> On 11/29/2024 9:53 PM, Dmitry Baryshkov wrote:
> > On Fri, 29 Nov 2024 at 09:59, Xiangxu Yin <quic_xiang...@quicinc.com> wrote:
> >>
> >> Add a mechanism to retry Link Training 2 by lowering the pattern level
> >> when the link training #2 first attempt fails. This approach enhances
> >> compatibility, particularly addressing issues caused by certain hub
> >> configurations.
> >
> > Please reference corresponding part of the standard, describing this
> > lowering.
> >
> Per DisplayPort 1.4a specification Section 3.5.1.2 and Table 3-10, while the
> standard doesn't explicitly define a TPS downgrade mechanism, it does specify:
Anything in DP 2.1?
> - All devices shall support TPS1 and TPS2
> - HDR2-capable devices shall support TPS3
> - HDR3-capable devices shall support TPS4
> While these capabilities are explicitly defined DPCD for sink devices, source
> device capabilities are less strictly defined, with the minimum requirement
> being support for TPS1 and TPS2.
> In QCS615 DP phy is only supporting to HBR2, we observed a critical
> interoperability scenario with a DP->HDMI bridge. When link training at TPS4
> consistently failed, downgrading to the next lower training pattern
> successfully established the link and display output successfully.
Any other driver doing such TPS lowering? Or maybe we should be
selecting TPS3 for HBR2-only devices?
>
> This experience suggests that implementing a flexible link training pattern
> downgrade mechanism can significantly improve compatibility with third-party,
> non-standard hubs and displays,
> especially in scenarios where strict adherence to the highest training
> pattern might prevent successful connection.
> >>
> >> Signed-off-by: Xiangxu Yin <quic_xiang...@quicinc.com>
> >> ---
> >> drivers/gpu/drm/msm/dp/dp_ctrl.c | 34 ++++++++++++++++++++++++++++++----
> >> 1 file changed, 30 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/msm/dp/dp_ctrl.c
> >> b/drivers/gpu/drm/msm/dp/dp_ctrl.c
> >> index
> >> 49c8ce9b2d0e57a613e50865be3fe98e814d425a..b1862294cb98c9f756b0108b7670cb42de37bae4
> >> 100644
> >> --- a/drivers/gpu/drm/msm/dp/dp_ctrl.c
> >> +++ b/drivers/gpu/drm/msm/dp/dp_ctrl.c
> >> @@ -1220,7 +1220,7 @@ static void
> >> msm_dp_ctrl_clear_training_pattern(struct msm_dp_ctrl_private *ctrl)
> >> }
> >>
> >> static int msm_dp_ctrl_link_train_2(struct msm_dp_ctrl_private *ctrl,
> >> - int *training_step)
> >> + int *training_step, bool downgrade)
> >> {
> >> int tries = 0, ret = 0;
> >> u8 pattern;
> >> @@ -1243,6 +1243,28 @@ static int msm_dp_ctrl_link_train_2(struct
> >> msm_dp_ctrl_private *ctrl,
> >> state_ctrl_bit = 2;
> >> }
> >>
> >> + /*
> >> + * DP link training uses the highest allowed pattern by default.
> >> + * If it fails, the pattern is downgraded to improve cable and
> >> monitor compatibility.
> >> + */
> >> + if (downgrade) {
> >> + switch (pattern) {
> >> + case DP_TRAINING_PATTERN_4:
> >> + pattern = DP_TRAINING_PATTERN_3;
> >> + state_ctrl_bit = 3;
> >> + break;
> >> + case DP_TRAINING_PATTERN_3:
> >> + pattern = DP_TRAINING_PATTERN_2;
> >> + state_ctrl_bit = 2;
> >> + break;
> >> + default:
> >> + break;
> >> + }
> >> + }
> >> +
> >> + drm_dbg_dp(ctrl->drm_dev, "pattern(%d) state_ctrl_bit(%d)
> >> downgrade(%d)\n",
> >> + pattern, state_ctrl_bit, downgrade);
> >> +
> >> ret = msm_dp_catalog_ctrl_set_pattern_state_bit(ctrl->catalog,
> >> state_ctrl_bit);
> >> if (ret)
> >> return ret;
> >> @@ -1311,10 +1333,14 @@ static int msm_dp_ctrl_link_train(struct
> >> msm_dp_ctrl_private *ctrl,
> >> /* print success info as this is a result of user initiated action
> >> */
> >> drm_dbg_dp(ctrl->drm_dev, "link training #1 successful\n");
> >>
> >> - ret = msm_dp_ctrl_link_train_2(ctrl, training_step);
> >> + ret = msm_dp_ctrl_link_train_2(ctrl, training_step, false);
> >> if (ret) {
> >> - DRM_ERROR("link training #2 failed. ret=%d\n", ret);
> >> - goto end;
> >> + drm_dbg_dp(ctrl->drm_dev, "link training #2 failed, retry
> >> downgrade.\n");
> >> + ret = msm_dp_ctrl_link_train_2(ctrl, training_step, true);
> >> + if (ret) {
> >> + DRM_ERROR("link training #2 failed. ret=%d\n",
> >> ret);
> >> + goto end;
> >> + }
> >> }
> >>
> >> /* print success info as this is a result of user initiated action
> >> */
> >>
> >> --
> >> 2.25.1
> >>
> >
> >
>
>
> --
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--
With best wishes
Dmitry
