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>

diff --git a/drivers/spi/spi-pl022.c b/drivers/spi/spi-pl022.c
index 2f9cb43..81847c9 100644 (file)
--- 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;

diff --git a/include/linux/amba/pl022.h b/include/linux/amba/pl022.h
index 572f637..3672f40 100644 (file)
--- a/include/linux/amba/pl022.h
+++ b/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;
 };
 
 /**