[PATCH] drm/dp_mst: Have DP_Tx send one msg at a time

[Lyude Paul]

Reviewed-by: Lyude Paul <lyude at redhat.com>

I will push this to drm-misc-fixes in just a moment, thanks!

On Mon, 2020-01-13 at 17:36 +0800, Wayne Lin wrote:
> [Why]
> Noticed this while testing MST with the 4 ports MST hub from
> StarTech.com. Sometimes can't light up monitors normally and get the
> error message as 'sideband msg build failed'.
> 
> Look into aux transactions, found out that source sometimes will send
> out another down request before receiving the down reply of the
> previous down request. On the other hand, in drm_dp_get_one_sb_msg(),
> current code doesn't handle the interleaved replies case. Hence, source
> can't build up message completely and can't light up monitors.
> 
> [How]
> For good compatibility, enforce source to send out one down request at a
> time. Add a flag, is_waiting_for_dwn_reply, to determine if the source
> can send out a down request immediately or not.
> 
> - Check the flag before calling process_single_down_tx_qlock to send out
> a msg
> - Set the flag when successfully send out a down request
> - Clear the flag when successfully build up a down reply
> - Clear the flag when find erros during sending out a down request
> - Clear the flag when find errors during building up a down reply
> - Clear the flag when timeout occurs during waiting for a down reply
> - Use drm_dp_mst_kick_tx() to try to send another down request in queue
> at the end of drm_dp_mst_wait_tx_reply() (attempt to send out messages
> in queue when errors occur)
> 
> Cc: Lyude Paul <lyude at redhat.com>
> Signed-off-by: Wayne Lin <Wayne.Lin at amd.com>
> ---
>  drivers/gpu/drm/drm_dp_mst_topology.c | 14 ++++++++++++--
>  include/drm/drm_dp_mst_helper.h       |  6 ++++++
>  2 files changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c
> b/drivers/gpu/drm/drm_dp_mst_topology.c
> index 4395d5cc0645..3542af15387a 100644
> --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> @@ -1195,6 +1195,8 @@ static int drm_dp_mst_wait_tx_reply(struct
> drm_dp_mst_branch *mstb,
>  		    txmsg->state == DRM_DP_SIDEBAND_TX_SENT) {
>  			mstb->tx_slots[txmsg->seqno] = NULL;
>  		}
> +		mgr->is_waiting_for_dwn_reply = false;
> +
>  	}
>  out:
>  	if (unlikely(ret == -EIO) && drm_debug_enabled(DRM_UT_DP)) {
> @@ -1204,6 +1206,7 @@ static int drm_dp_mst_wait_tx_reply(struct
> drm_dp_mst_branch *mstb,
>  	}
>  	mutex_unlock(&mgr->qlock);
>  
> +	drm_dp_mst_kick_tx(mgr);
>  	return ret;
>  }
>  
> @@ -2770,9 +2773,11 @@ static void process_single_down_tx_qlock(struct
> drm_dp_mst_topology_mgr *mgr)
>  	ret = process_single_tx_qlock(mgr, txmsg, false);
>  	if (ret == 1) {
>  		/* txmsg is sent it should be in the slots now */
> +		mgr->is_waiting_for_dwn_reply = true;
>  		list_del(&txmsg->next);
>  	} else if (ret) {
>  		DRM_DEBUG_KMS("failed to send msg in q %d\n", ret);
> +		mgr->is_waiting_for_dwn_reply = false;
>  		list_del(&txmsg->next);
>  		if (txmsg->seqno != -1)
>  			txmsg->dst->tx_slots[txmsg->seqno] = NULL;
> @@ -2812,7 +2817,8 @@ static void drm_dp_queue_down_tx(struct
> drm_dp_mst_topology_mgr *mgr,
>  		drm_dp_mst_dump_sideband_msg_tx(&p, txmsg);
>  	}
>  
> -	if (list_is_singular(&mgr->tx_msg_downq))
> +	if (list_is_singular(&mgr->tx_msg_downq) &&
> +	    !mgr->is_waiting_for_dwn_reply)
>  		process_single_down_tx_qlock(mgr);
>  	mutex_unlock(&mgr->qlock);
>  }
> @@ -3743,6 +3749,7 @@ static int drm_dp_mst_handle_down_rep(struct
> drm_dp_mst_topology_mgr *mgr)
>  	mutex_lock(&mgr->qlock);
>  	txmsg->state = DRM_DP_SIDEBAND_TX_RX;
>  	mstb->tx_slots[slot] = NULL;
> +	mgr->is_waiting_for_dwn_reply = false;
>  	mutex_unlock(&mgr->qlock);
>  
>  	wake_up_all(&mgr->tx_waitq);
> @@ -3752,6 +3759,9 @@ static int drm_dp_mst_handle_down_rep(struct
> drm_dp_mst_topology_mgr *mgr)
>  no_msg:
>  	drm_dp_mst_topology_put_mstb(mstb);
>  clear_down_rep_recv:
> +	mutex_lock(&mgr->qlock);
> +	mgr->is_waiting_for_dwn_reply = false;
> +	mutex_unlock(&mgr->qlock);
>  	memset(&mgr->down_rep_recv, 0, sizeof(struct drm_dp_sideband_msg_rx));
>  
>  	return 0;
> @@ -4591,7 +4601,7 @@ static void drm_dp_tx_work(struct work_struct *work)
>  	struct drm_dp_mst_topology_mgr *mgr = container_of(work, struct
> drm_dp_mst_topology_mgr, tx_work);
>  
>  	mutex_lock(&mgr->qlock);
> -	if (!list_empty(&mgr->tx_msg_downq))
> +	if (!list_empty(&mgr->tx_msg_downq) && !mgr->is_waiting_for_dwn_reply)
>  		process_single_down_tx_qlock(mgr);
>  	mutex_unlock(&mgr->qlock);
>  }
> diff --git a/include/drm/drm_dp_mst_helper.h
> b/include/drm/drm_dp_mst_helper.h
> index 942575de86a0..d0b45468135a 100644
> --- a/include/drm/drm_dp_mst_helper.h
> +++ b/include/drm/drm_dp_mst_helper.h
> @@ -610,6 +610,12 @@ struct drm_dp_mst_topology_mgr {
>  	 * &drm_dp_sideband_msg_tx.state once they are queued
>  	 */
>  	struct mutex qlock;
> +
> +	/**
> +	 * @is_waiting_for_dwn_reply: indicate whether is waiting for down
> reply
> +	 */
> +	bool is_waiting_for_dwn_reply;
> +
>  	/**
>  	 * @tx_msg_downq: List of pending down replies.
>  	 */
-- 
Cheers,
	Lyude Paul