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SubjectRe: [PATCH 2/2] [RFC] spi/bitbang: Use a kthread worker instead of workqueue
On Tue, Feb 08, 2011 at 10:46:15AM +0100, Uwe Kleine-König wrote:
> From: Sascha Hauer <s.hauer@pengutronix.de>
>
> Since the concurrency managed workqueues are merged we often have
> several microseconds delays in spi transfers. This patch resolves
> this by using a kthread worker in spi_bitbang instead.
>
> Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
> Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

Hmmm, I could use some input here from folks with a better
understanding of the concurrency managed workqueues. Is switching to
a kthread worker the right thing to do, or is it working around a bug
in the workqueue implementation that should be fixed instead?

Anyone care to chime in?

g.

> ---
> drivers/spi/spi_bitbang.c | 17 ++++++++++-------
> include/linux/spi/spi_bitbang.h | 7 ++++---
> 2 files changed, 14 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/spi/spi_bitbang.c b/drivers/spi/spi_bitbang.c
> index 14a63f6..e2a874a 100644
> --- a/drivers/spi/spi_bitbang.c
> +++ b/drivers/spi/spi_bitbang.c
> @@ -254,7 +254,7 @@ static int spi_bitbang_bufs(struct spi_device *spi, struct spi_transfer *t)
> * Drivers can provide word-at-a-time i/o primitives, or provide
> * transfer-at-a-time ones to leverage dma or fifo hardware.
> */
> -static void bitbang_work(struct work_struct *work)
> +static void bitbang_work(struct kthread_work *work)
> {
> struct spi_bitbang *bitbang =
> container_of(work, struct spi_bitbang, work);
> @@ -396,7 +396,7 @@ int spi_bitbang_transfer(struct spi_device *spi, struct spi_message *m)
> status = -ENETDOWN;
> else {
> list_add_tail(&m->queue, &bitbang->queue);
> - queue_work(bitbang->workqueue, &bitbang->work);
> + queue_kthread_work(&bitbang->worker, &bitbang->work);
> }
> spin_unlock_irqrestore(&bitbang->lock, flags);
>
> @@ -432,11 +432,12 @@ EXPORT_SYMBOL_GPL(spi_bitbang_transfer);
> int spi_bitbang_start(struct spi_bitbang *bitbang)
> {
> int status;
> + struct sched_param param = { .sched_priority = 49 };
>
> if (!bitbang->master || !bitbang->chipselect)
> return -EINVAL;
>
> - INIT_WORK(&bitbang->work, bitbang_work);
> + init_kthread_work(&bitbang->work, bitbang_work);
> spin_lock_init(&bitbang->lock);
> INIT_LIST_HEAD(&bitbang->queue);
>
> @@ -463,12 +464,14 @@ int spi_bitbang_start(struct spi_bitbang *bitbang)
>
> /* this task is the only thing to touch the SPI bits */
> bitbang->busy = 0;
> - bitbang->workqueue = create_singlethread_workqueue(
> + init_kthread_worker(&bitbang->worker);
> + bitbang->worker_task = kthread_run(kthread_worker_fn, &bitbang->worker,
> dev_name(bitbang->master->dev.parent));
> - if (bitbang->workqueue == NULL) {
> + if (bitbang->worker_task == NULL) {
> status = -EBUSY;
> goto err1;
> }
> + sched_setscheduler(bitbang->worker_task, SCHED_FIFO, &param);
>
> /* driver may get busy before register() returns, especially
> * if someone registered boardinfo for devices
> @@ -480,7 +483,7 @@ int spi_bitbang_start(struct spi_bitbang *bitbang)
> return status;
>
> err2:
> - destroy_workqueue(bitbang->workqueue);
> + kthread_stop(bitbang->worker_task);
> err1:
> return status;
> }
> @@ -495,7 +498,7 @@ int spi_bitbang_stop(struct spi_bitbang *bitbang)
>
> WARN_ON(!list_empty(&bitbang->queue));
>
> - destroy_workqueue(bitbang->workqueue);
> + kthread_stop(bitbang->worker_task);
>
> return 0;
> }
> diff --git a/include/linux/spi/spi_bitbang.h b/include/linux/spi/spi_bitbang.h
> index f987a2b..36bb3be 100644
> --- a/include/linux/spi/spi_bitbang.h
> +++ b/include/linux/spi/spi_bitbang.h
> @@ -1,11 +1,12 @@
> #ifndef __SPI_BITBANG_H
> #define __SPI_BITBANG_H
>
> -#include <linux/workqueue.h>
> +#include <linux/kthread.h>
>
> struct spi_bitbang {
> - struct workqueue_struct *workqueue;
> - struct work_struct work;
> + struct kthread_worker worker;
> + struct task_struct *worker_task;
> + struct kthread_work work;
>
> spinlock_t lock;
> struct list_head queue;
> --
> 1.7.2.3
>
>
> ------------------------------------------------------------------------------
> The ultimate all-in-one performance toolkit: Intel(R) Parallel Studio XE:
> Pinpoint memory and threading errors before they happen.
> Find and fix more than 250 security defects in the development cycle.
> Locate bottlenecks in serial and parallel code that limit performance.
> http://p.sf.net/sfu/intel-dev2devfeb
> _______________________________________________
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> spi-devel-general@lists.sourceforge.net
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