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>

1 files changed, 52 insertions, 25 deletions

diff --git a/drivers/spi/spi-pl022.c b/drivers/spi/spi-pl022.c
index 2f9cb43a2398..81847c9a7586 100644
--- a/drivers/spi/spi-pl022.c
+++ b/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;
@@ -1494,8 +1497,8 @@ out:
 }
 
 /**
- * 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
@@ -1503,7 +1506,7 @@ out:
  * 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;