spi/pl022: Add high priority message pump support

This switches the PL022 worker to a kthread in order to get
hold of a mechanism to control the message pump priority. On
low-latency systems elevating the message kthread to realtime
priority give a real sleek response curve. This has been
confirmed by measurements. Realtime priority elevation for
a certain PL022 port can be requested from platform data.

Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Chris Blair <chris.blair@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

drivers/spi/spi-pl022.c

@@ -29,7 +29,7 @@
 #include <linux/errno.h>
 #include <linux/interrupt.h>
 #include <linux/spi/spi.h>
-#include <linux/workqueue.h>
+#include <linux/kthread.h>
 #include <linux/delay.h>
 #include <linux/clk.h>
 #include <linux/err.h>
@@ -41,6 +41,7 @@
 #include <linux/dma-mapping.h>
 #include <linux/scatterlist.h>
 #include <linux/pm_runtime.h>
+#include <linux/sched.h>
 
 /*
  * This macro is used to define some register default values.
@@ -330,12 +331,13 @@ struct vendor_data {
  * @clk: outgoing clock "SPICLK" for the SPI bus
  * @master: SPI framework hookup
  * @master_info: controller-specific data from machine setup
- * @workqueue: a workqueue on which any spi_message request is queued
- * @pump_messages: work struct for scheduling work to the workqueue
+ * @kworker: thread struct for message pump
+ * @kworker_task: pointer to task for message pump kworker thread
+ * @pump_messages: work struct for scheduling work to the message pump
  * @queue_lock: spinlock to syncronise access to message queue
  * @queue: message queue
- * @busy: workqueue is busy
- * @running: workqueue is running
+ * @busy: message pump is busy
+ * @running: message pump is running
  * @pump_transfers: Tasklet used in Interrupt Transfer mode
  * @cur_msg: Pointer to current spi_message being processed
  * @cur_transfer: Pointer to current spi_transfer
@@ -365,9 +367,10 @@ struct pl022 {
 	struct clk			*clk;
 	struct spi_master		*master;
 	struct pl022_ssp_controller	*master_info;
-	/* Driver message queue */
-	struct workqueue_struct		*workqueue;
-	struct work_struct		pump_messages;
+	/* Driver message pump */
+	struct kthread_worker		kworker;
+	struct task_struct		*kworker_task;
+	struct kthread_work		pump_messages;
 	spinlock_t			queue_lock;
 	struct list_head		queue;
 	bool				busy;
@@ -504,7 +507,7 @@ static void giveback(struct pl022 *pl022)
 	pl022->cur_msg = NULL;
 	pl022->cur_transfer = NULL;
 	pl022->cur_chip = NULL;
-	queue_work(pl022->workqueue, &pl022->pump_messages);
+	queue_kthread_work(&pl022->kworker, &pl022->pump_messages);
 	spin_unlock_irqrestore(&pl022->queue_lock, flags);
 
 	msg->state = NULL;
@@ -1494,16 +1497,16 @@ static void do_polling_transfer(struct pl022 *pl022)
 }
 
 /**
- * pump_messages - Workqueue function which processes spi message queue
- * @data: pointer to private data of SSP driver
+ * pump_messages - kthread work function which processes spi message queue
+ * @work: pointer to kthread work struct contained in the pl022 private struct
  *
  * This function checks if there is any spi message in the queue that
  * needs processing and delegate control to appropriate function
  * do_polling_transfer()/do_interrupt_dma_transfer()
  * based on the kind of the transfer
  *
  */
-static void pump_messages(struct work_struct *work)
+static void pump_messages(struct kthread_work *work)
 {
 	struct pl022 *pl022 =
 		container_of(work, struct pl022, pump_messages);
@@ -1556,7 +1559,7 @@ static void pump_messages(struct work_struct *work)
 	if (!was_busy)
 		/*
 		 * We enable the core voltage and clocks here, then the clocks
-		 * and core will be disabled when this workqueue is run again
+		 * and core will be disabled when this thread is run again
 		 * and there is no more work to be done.
 		 */
 		pm_runtime_get_sync(&pl022->adev->dev);
@@ -1572,6 +1575,8 @@ static void pump_messages(struct work_struct *work)
 
 static int __init init_queue(struct pl022 *pl022)
 {
+	struct sched_param param = { .sched_priority = MAX_RT_PRIO - 1 };
+
 	INIT_LIST_HEAD(&pl022->queue);
 	spin_lock_init(&pl022->queue_lock);
 
@@ -1581,11 +1586,29 @@ static int __init init_queue(struct pl022 *pl022)
 	tasklet_init(&pl022->pump_transfers, pump_transfers,
 			(unsigned long)pl022);
 
-	INIT_WORK(&pl022->pump_messages, pump_messages);
-	pl022->workqueue = create_singlethread_workqueue(
+	init_kthread_worker(&pl022->kworker);
+	pl022->kworker_task = kthread_run(kthread_worker_fn,
+					&pl022->kworker,
 					dev_name(pl022->master->dev.parent));
-	if (pl022->workqueue == NULL)
-		return -EBUSY;
+	if (IS_ERR(pl022->kworker_task)) {
+		dev_err(&pl022->adev->dev,
+			"failed to create message pump task\n");
+		return -ENOMEM;
+	}
+	init_kthread_work(&pl022->pump_messages, pump_messages);
+
+	/*
+	 * Board config will indicate if this controller should run the
+	 * message pump with high (realtime) priority to reduce the transfer
+	 * latency on the bus by minimising the delay between a transfer
+	 * request and the scheduling of the message pump thread. Without this
+	 * setting the message pump thread will remain at default priority.
+	 */
+	if (pl022->master_info->rt) {
+		dev_info(&pl022->adev->dev,
+			"will run message pump with realtime priority\n");
+		sched_setscheduler(pl022->kworker_task, SCHED_FIFO, &param);
+	}
 
 	return 0;
 }
@@ -1608,7 +1631,7 @@ static int start_queue(struct pl022 *pl022)
 	pl022->next_msg_cs_active = false;
 	spin_unlock_irqrestore(&pl022->queue_lock, flags);
 
-	queue_work(pl022->workqueue, &pl022->pump_messages);
+	queue_kthread_work(&pl022->kworker, &pl022->pump_messages);
 
 	return 0;
 }
@@ -1646,16 +1669,20 @@ static int destroy_queue(struct pl022 *pl022)
 	int status;
 
 	status = stop_queue(pl022);
-	/* we are unloading the module or failing to load (only two calls
+
+	/*
+	 * We are unloading the module or failing to load (only two calls
 	 * to this routine), and neither call can handle a return value.
-	 * However, destroy_workqueue calls flush_workqueue, and that will
-	 * block until all work is done.  If the reason that stop_queue
-	 * timed out is that the work will never finish, then it does no
-	 * good to call destroy_workqueue, so return anyway. */
+	 * However, flush_kthread_worker will block until all work is done.
+	 * If the reason that stop_queue timed out is that the work will never
+	 * finish, then it does no good to call flush/stop thread, so
+	 * return anyway.
+	 */
 	if (status != 0)
 		return status;
 
-	destroy_workqueue(pl022->workqueue);
+	flush_kthread_worker(&pl022->kworker);
+	kthread_stop(pl022->kworker_task);
 
 	return 0;
 }
@@ -1802,7 +1829,7 @@ static int pl022_transfer(struct spi_device *spi, struct spi_message *msg)
 
 	list_add_tail(&msg->queue, &pl022->queue);
 	if (pl022->running && !pl022->busy)
-		queue_work(pl022->workqueue, &pl022->pump_messages);
+		queue_kthread_work(&pl022->kworker, &pl022->pump_messages);
 
 	spin_unlock_irqrestore(&pl022->queue_lock, flags);
 	return 0;

include/linux/amba/pl022.h

@@ -241,6 +241,8 @@ struct dma_chan;
  * @autosuspend_delay: delay in ms following transfer completion before the
  *     runtime power management system suspends the device. A setting of 0
  *     indicates no delay and the device will be suspended immediately.
+ * @rt: indicates the controller should run the message pump with realtime
+ *     priority to minimise the transfer latency on the bus.
  */
 struct pl022_ssp_controller {
 	u16 bus_id;
@@ -250,6 +252,7 @@ struct pl022_ssp_controller {
 	void *dma_rx_param;
 	void *dma_tx_param;
 	int autosuspend_delay;
+	bool rt;
 };
 
 /**