[PATCH 1/2] drm/display/dp: Check for MSTM_CAP before MSTM_CTRL write

[Lin, Charlton]

On 1/27/2024 12:06 PM, Arun R Murthy wrote:
> With DP2.1, multistream packetization and the underneth MST protocol
> will be required for SST. So check for MSTM_CAP to see if MST is really
> required and skip the MSTM_CTRL write so that we ensure that only the
> underneth protocol and the multistream packetization will be enabled and
> sink will not be confused by a corresponding dpcd write.
>
> Signed-off-by: Arun R Murthy <arun.r.murthy at intel.com>
> ---
>  drivers/gpu/drm/display/drm_dp_mst_topology.c | 26 +++++++++++--------
>  1 file changed, 15 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> index 8ca01a6bf645..22d81732a978 100644
> --- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> @@ -3666,10 +3666,11 @@ int drm_dp_mst_topology_mgr_set_mst(struct drm_dp_mst_topology_mgr *mgr, bool ms
>                           mgr->mst_primary = mstb;
>                           drm_dp_mst_topology_get_mstb(mgr->mst_primary);
>
> -                        ret = drm_dp_dpcd_writeb(mgr->aux, DP_MSTM_CTRL,
> -                                                                   DP_MST_EN |
> -                                                                   DP_UP_REQ_EN |
> -                                                                   DP_UPSTREAM_IS_SRC);
> +                       if (drm_dp_read_mst_cap(mgr->aux, mgr->dpcd))
> +                                      ret = drm_dp_dpcd_writeb(mgr->aux, DP_MSTM_CTRL,
> +                                                                                 DP_MST_EN |
> +                                                                                 DP_UP_REQ_EN |
> +                                                                                 DP_UPSTREAM_IS_SRC);
>                           if (ret < 0)
>                                         goto out_unlock;
>
> @@ -3684,7 +3685,8 @@ int drm_dp_mst_topology_mgr_set_mst(struct drm_dp_mst_topology_mgr *mgr, bool ms
>                           mstb = mgr->mst_primary;
>                           mgr->mst_primary = NULL;
>                           /* this can fail if the device is gone */
> -                        drm_dp_dpcd_writeb(mgr->aux, DP_MSTM_CTRL, 0);
> +                       if (drm_dp_read_mst_cap(mgr->aux, mgr->dpcd))
> +                                      drm_dp_dpcd_writeb(mgr->aux, DP_MSTM_CTRL, 0);
>                           ret = 0;
>                           mgr->payload_id_table_cleared = false;
>
> @@ -3724,8 +3726,9 @@ drm_dp_mst_topology_mgr_invalidate_mstb(struct drm_dp_mst_branch *mstb)
>  void drm_dp_mst_topology_mgr_suspend(struct drm_dp_mst_topology_mgr *mgr)
>  {
>            mutex_lock(&mgr->lock);
> -          drm_dp_dpcd_writeb(mgr->aux, DP_MSTM_CTRL,
> -                                          DP_MST_EN | DP_UPSTREAM_IS_SRC);
> +         if (drm_dp_read_mst_cap(mgr->aux, mgr->dpcd))
> +                       drm_dp_dpcd_writeb(mgr->aux, DP_MSTM_CTRL,
> +                                                       DP_MST_EN | DP_UPSTREAM_IS_SRC);
>            mutex_unlock(&mgr->lock);
>            flush_work(&mgr->up_req_work);
>            flush_work(&mgr->work);
> @@ -3773,10 +3776,11 @@ int drm_dp_mst_topology_mgr_resume(struct drm_dp_mst_topology_mgr *mgr,
>                           goto out_fail;
>            }
>
> -          ret = drm_dp_dpcd_writeb(mgr->aux, DP_MSTM_CTRL,
> -                                                     DP_MST_EN |
> -                                                     DP_UP_REQ_EN |
> -                                                     DP_UPSTREAM_IS_SRC);
> +         if (drm_dp_read_mst_cap(mgr->aux, mgr->dpcd))
> +                       ret = drm_dp_dpcd_writeb(mgr->aux, DP_MSTM_CTRL,

This did not compile in Chrome dev environemnt due to uninitialized variable ret.

> +                                                                   DP_MST_EN |
> +                                                                   DP_UP_REQ_EN |
> +                                                                   DP_UPSTREAM_IS_SRC);
>            if (ret < 0) {
>                           drm_dbg_kms(mgr->dev, "mst write failed - undocked during suspend?\n");
>                           goto out_fail;

Tested-by: Charlton Lin <charlton.lin at intel.com>

Regards,
Charlton Lin
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