Re: [PATCH 3/6] drm: zynqmp_dp: Add locking

[Laurent Pinchart]

Hi Sean,

Thank you for the patch.

On Fri, Mar 15, 2024 at 07:09:13PM -0400, Sean Anderson wrote:
> Add some locking, since none is provided by the drm subsystem. This will

That's not quite right, the DRM core doesn't call bridge operations
concurrently. We may need locking to protect against race conditions
between bridge operations and interrupts though.

> prevent the IRQ/workers/bridge API calls from stepping on each other's
> toes.
> 
> Signed-off-by: Sean Anderson <sean.anderson@xxxxxxxxx>
> ---
> 
>  drivers/gpu/drm/xlnx/zynqmp_dp.c | 59 +++++++++++++++++++++++---------
>  1 file changed, 42 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xlnx/zynqmp_dp.c b/drivers/gpu/drm/xlnx/zynqmp_dp.c
> index 8635b5673386..d2dee58e7bf2 100644
> --- a/drivers/gpu/drm/xlnx/zynqmp_dp.c
> +++ b/drivers/gpu/drm/xlnx/zynqmp_dp.c
> @@ -279,6 +279,7 @@ struct zynqmp_dp_config {
>   * @dpsub: Display subsystem
>   * @iomem: device I/O memory for register access
>   * @reset: reset controller
> + * @lock: Mutex protecting this struct and register access (but not AUX)

This patch does two things at once, it defers link training from the IRQ
handler to a work queue, and covers everything with a big lock. The
scope is too large. Please restrict the lock scope and document the
individual fields that need to be protected, and explain the locking
design in the commit message (or comments in the code).

>   * @irq: irq
>   * @bridge: DRM bridge for the DP encoder
>   * @next_bridge: The downstream bridge
> @@ -299,6 +300,7 @@ struct zynqmp_dp {
>  	struct zynqmp_dpsub *dpsub;
>  	void __iomem *iomem;
>  	struct reset_control *reset;
> +	struct mutex lock;
>  	int irq;
>  
>  	struct drm_bridge bridge;
> @@ -308,7 +310,7 @@ struct zynqmp_dp {
>  	struct drm_dp_aux aux;
>  	struct phy *phy[ZYNQMP_DP_MAX_LANES];
>  	u8 num_lanes;
> -	struct delayed_work hpd_work;
> +	struct delayed_work hpd_work, hpd_irq_work;

One variable per line please.

>  	enum drm_connector_status status;
>  	bool enabled;
>  
> @@ -1371,8 +1373,10 @@ zynqmp_dp_bridge_mode_valid(struct drm_bridge *bridge,
>  	}
>  
>  	/* Check with link rate and lane count */
> +	mutex_lock(&dp->lock);
>  	rate = zynqmp_dp_max_rate(dp->link_config.max_rate,
>  				  dp->link_config.max_lanes, dp->config.bpp);
> +	mutex_unlock(&dp->lock);
>  	if (mode->clock > rate) {
>  		dev_dbg(dp->dev, "filtered mode %s for high pixel rate\n",
>  			mode->name);
> @@ -1399,6 +1403,7 @@ static void zynqmp_dp_bridge_atomic_enable(struct drm_bridge *bridge,
>  
>  	pm_runtime_get_sync(dp->dev);
>  
> +	mutex_lock(&dp->lock);
>  	zynqmp_dp_disp_enable(dp, old_bridge_state);
>  
>  	/*
> @@ -1459,6 +1464,7 @@ static void zynqmp_dp_bridge_atomic_enable(struct drm_bridge *bridge,
>  	zynqmp_dp_write(dp, ZYNQMP_DP_SOFTWARE_RESET,
>  			ZYNQMP_DP_SOFTWARE_RESET_ALL);
>  	zynqmp_dp_write(dp, ZYNQMP_DP_MAIN_STREAM_ENABLE, 1);
> +	mutex_unlock(&dp->lock);
>  }
>  
>  static void zynqmp_dp_bridge_atomic_disable(struct drm_bridge *bridge,
> @@ -1466,6 +1472,7 @@ static void zynqmp_dp_bridge_atomic_disable(struct drm_bridge *bridge,
>  {
>  	struct zynqmp_dp *dp = bridge_to_dp(bridge);
>  
> +	mutex_lock(&dp->lock);
>  	dp->enabled = false;
>  	cancel_delayed_work(&dp->hpd_work);
>  	zynqmp_dp_write(dp, ZYNQMP_DP_MAIN_STREAM_ENABLE, 0);
> @@ -1476,6 +1483,7 @@ static void zynqmp_dp_bridge_atomic_disable(struct drm_bridge *bridge,
>  		zynqmp_dp_write(dp, ZYNQMP_DP_TX_AUDIO_CONTROL, 0);
>  
>  	zynqmp_dp_disp_disable(dp, old_bridge_state);
> +	mutex_unlock(&dp->lock);
>  
>  	pm_runtime_put_sync(dp->dev);
>  }
> @@ -1518,6 +1526,8 @@ static enum drm_connector_status zynqmp_dp_bridge_detect(struct drm_bridge *brid
>  	u32 state, i;
>  	int ret;
>  
> +	mutex_lock(&dp->lock);
> +
>  	/*
>  	 * This is from heuristic. It takes some delay (ex, 100 ~ 500 msec) to
>  	 * get the HPD signal with some monitors.
> @@ -1545,11 +1555,13 @@ static enum drm_connector_status zynqmp_dp_bridge_detect(struct drm_bridge *brid
>  					       dp->num_lanes);
>  
>  		dp->status = connector_status_connected;
> +		mutex_unlock(&dp->lock);
>  		return connector_status_connected;
>  	}
>  
>  disconnected:
>  	dp->status = connector_status_disconnected;
> +	mutex_unlock(&dp->lock);
>  	return connector_status_disconnected;
>  }
>  
> @@ -1611,6 +1623,29 @@ static void zynqmp_dp_hpd_work_func(struct work_struct *work)
>  	drm_bridge_hpd_notify(&dp->bridge, status);
>  }
>  
> +static void zynqmp_dp_hpd_irq_work_func(struct work_struct *work)
> +{
> +	struct zynqmp_dp *dp = container_of(work, struct zynqmp_dp,
> +					    hpd_irq_work.work);
> +	u8 status[DP_LINK_STATUS_SIZE + 2];
> +	int err;
> +
> +	mutex_lock(&dp->lock);
> +	err = drm_dp_dpcd_read(&dp->aux, DP_SINK_COUNT, status,
> +			       DP_LINK_STATUS_SIZE + 2);
> +	if (err < 0) {
> +		dev_dbg_ratelimited(dp->dev,
> +				    "could not read sink status: %d\n", err);
> +	} else {
> +		if (status[4] & DP_LINK_STATUS_UPDATED ||
> +		    !drm_dp_clock_recovery_ok(&status[2], dp->mode.lane_cnt) ||
> +		    !drm_dp_channel_eq_ok(&status[2], dp->mode.lane_cnt)) {
> +			zynqmp_dp_train_loop(dp);
> +		}
> +	}
> +	mutex_unlock(&dp->lock);
> +}
> +
>  static irqreturn_t zynqmp_dp_irq_handler(int irq, void *data)
>  {
>  	struct zynqmp_dp *dp = (struct zynqmp_dp *)data;
> @@ -1635,23 +1670,9 @@ static irqreturn_t zynqmp_dp_irq_handler(int irq, void *data)
>  	if (status & ZYNQMP_DP_INT_HPD_EVENT)
>  		schedule_delayed_work(&dp->hpd_work, 0);
>  
> -	if (status & ZYNQMP_DP_INT_HPD_IRQ) {
> -		int ret;
> -		u8 status[DP_LINK_STATUS_SIZE + 2];
> +	if (status & ZYNQMP_DP_INT_HPD_IRQ)
> +		schedule_delayed_work(&dp->hpd_irq_work, 0);
>  
> -		ret = drm_dp_dpcd_read(&dp->aux, DP_SINK_COUNT, status,
> -				       DP_LINK_STATUS_SIZE + 2);
> -		if (ret < 0)
> -			goto handled;
> -
> -		if (status[4] & DP_LINK_STATUS_UPDATED ||
> -		    !drm_dp_clock_recovery_ok(&status[2], dp->mode.lane_cnt) ||
> -		    !drm_dp_channel_eq_ok(&status[2], dp->mode.lane_cnt)) {
> -			zynqmp_dp_train_loop(dp);
> -		}
> -	}
> -
> -handled:
>  	return IRQ_HANDLED;
>  }
>  
> @@ -1674,8 +1695,10 @@ int zynqmp_dp_probe(struct zynqmp_dpsub *dpsub)
>  	dp->dev = &pdev->dev;
>  	dp->dpsub = dpsub;
>  	dp->status = connector_status_disconnected;
> +	mutex_init(&dp->lock);
>  
>  	INIT_DELAYED_WORK(&dp->hpd_work, zynqmp_dp_hpd_work_func);
> +	INIT_DELAYED_WORK(&dp->hpd_irq_work, zynqmp_dp_hpd_irq_work_func);
>  
>  	/* Acquire all resources (IOMEM, IRQ and PHYs). */
>  	res = platform_get_resource_byname(pdev, IORESOURCE_MEM, "dp");
> @@ -1775,6 +1798,7 @@ void zynqmp_dp_remove(struct zynqmp_dpsub *dpsub)
>  	zynqmp_dp_write(dp, ZYNQMP_DP_INT_DS, ZYNQMP_DP_INT_ALL);
>  	disable_irq(dp->irq);
>  
> +	cancel_delayed_work_sync(&dp->hpd_irq_work);
>  	cancel_delayed_work_sync(&dp->hpd_work);
>  
>  	zynqmp_dp_write(dp, ZYNQMP_DP_TRANSMITTER_ENABLE, 0);
> @@ -1782,4 +1806,5 @@ void zynqmp_dp_remove(struct zynqmp_dpsub *dpsub)
>  
>  	zynqmp_dp_phy_exit(dp);
>  	zynqmp_dp_reset(dp, true);
> +	mutex_destroy(&dp->lock);
>  }

-- 
Regards,

Laurent Pinchart