[RFC 0/4] Improving SPI driver latency (vs v3.8.13.14-rt31)

[RFC 0/4] Improving SPI driver latency (vs v3.8.13.14-rt31)

[Jeff Epler]

I recently became interested in realtime access to an SPI device on the
Odroid U3 platform, with a goal of running a repetitive task every 1ms
that performs two SPI transactions totalling around 200 bytes.  (for
http://linuxcnc.org/ interfacing to a http://mesanet.com/ "Anything I/O"
card)

Unfortunately, I found that there were frequent large latencies, some
over 10ms, when using /dev/spidev.  This seems to be typical of others'
experience using it (for instance, one can find threads discussing
disappointing RT performance of SPI on the beaglebone and pandaboard; at
least one Raspberry PI project chose to implement a pure userspace SPI
driver instead of using spidev)

At all levels of the SPI stack, I found things that could be improved if
lowest delays are the goal.  I doubt that in their current form these
changes are suitable to be incorporated in linux, but I hope that
this might spur some discussion that would ultimately lead to better
realtime performance of SPI particularly in the preempt-rt kernel.

As you may know, the kernel's spi driver stack consists of
    spidev    - implementation of the /dev/spidev* device
    spi       - hardware-independent kernel layer
    spi-xyzzy - hardware-dependent driver for xyzzy hardware
                (s3c64xx in my device)

I fixed causes of latency at each layer
 * First, I eliminated per-ioctl memory allocations in spidev
 * Second, I made __spi_sync *actually* synchronous, rather than
   being a wrapper over spi_async + wait_for_completion; and changed
   spidev to use spi_sync.  This is probably the most controversial
   aspect of my patches.
 * Third, in the hardware-dependent code I moved DMA acquisition to
   device initialization time rather than transaction time

With these changes, I have been able to run at a 500us repetitive rate,
performing two transactions per repetition, with no transaction over 250us,
for over 24 hours.  This is in contrast to the original performance, in
which transactions taking over 10ms were seen multiple times per hour.
(24 hours is about 340 million spi transactions)  This means there's plenty
of margin for the other activities LinuxCNC needs to perform while still
running at a 1ms repetitive rate.

I know that 3.8 is by no means current, but 3.8.y is the default kernel
shipped by hardkernel for their U3 devices so it was a rational version
choice for me.  I did skim spi changes from version 3.8 to the present
and didn't see anything that looked like it was directed at improving
SPI latency, though the underlying code has probably changed enough over
time that I assume my patches wouldn't actually apply at the tip of the
latest and greatest branches.

Jeff Epler (4):
  spi: reenable sync SPI transfers
  spidev: Avoid runtime memory allocations
  spidev: actually use synchronous transfers
  spidev-s3c64xx: allocate dma channel at startup

 drivers/spi/spi-s3c64xx.c | 15 +++++++--------
 drivers/spi/spi.c         | 22 ++++++++++++++++------
 drivers/spi/spidev.c      | 43 ++++++++++++++++++++++++++++++-------------
 3 files changed, 53 insertions(+), 27 deletions(-)

-- 
2.1.0

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[RFC 1/4] spi: reenable sync SPI transfers

[Jeff Epler]

one source of excess latency in hm2_spi / hal_spidev is latency due
to async transfers.  Make the __spi_sync primitive actually synchronous,
rather than building on an asynchronous primitive.
---
 drivers/spi/spi.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 19ee901..55404b5 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -664,10 +664,18 @@ void spi_finalize_current_message(struct spi_master *master)
 	struct spi_message *mesg;
 	unsigned long flags;
 
+        if(!master->cur_msg)
+                return;
+
 	spin_lock_irqsave(&master->queue_lock, flags);
 	mesg = master->cur_msg;
 	master->cur_msg = NULL;
 
+        if(!mesg) {
+                spin_unlock_irqrestore(&master->queue_lock, flags);
+                return;
+        }
+
 	queue_kthread_work(&master->kworker, &master->pump_messages);
 	spin_unlock_irqrestore(&master->queue_lock, flags);
 
@@ -1490,24 +1498,26 @@ static int __spi_sync(struct spi_device *spi, struct spi_message *message,
 		      int bus_locked)
 {
 	DECLARE_COMPLETION_ONSTACK(done);
-	int status;
+	int status = 0;
 	struct spi_master *master = spi->master;
 
 	message->complete = spi_complete;
 	message->context = &done;
+	message->spi = spi;
 
 	if (!bus_locked)
 		mutex_lock(&master->bus_lock_mutex);
 
-	status = spi_async_locked(spi, message);
+        if(master->prepare_transfer_hardware)
+            status = master->prepare_transfer_hardware(master);
+        if(status >= 0)
+            status = master->transfer_one_message(master, message);
+        if(status >= 0 && master->unprepare_transfer_hardware)
+            status = master->unprepare_transfer_hardware(master);
 
 	if (!bus_locked)
 		mutex_unlock(&master->bus_lock_mutex);
 
-	if (status == 0) {
-		wait_for_completion(&done);
-		status = message->status;
-	}
 	message->context = NULL;
 	return status;
 }
-- 
2.1.0

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Re: [RFC 1/4] spi: reenable sync SPI transfers

[Mark Brown]

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On Sun, Sep 14, 2014 at 09:45:05AM -0500, Jeff Epler wrote:
> one source of excess latency in hm2_spi / hal_spidev is latency due
> to async transfers.  Make the __spi_sync primitive actually synchronous,
> rather than building on an asynchronous primitive.
> ---

Please submit patches using the process that is documented in 
Documentation/SubmittingPatches, in particular it is *essential* that
you sign off your patches and you need CC maintainers otherwise it's
likely your patch will be missed.

> +        if(!master->cur_msg)
> +                return;
> +

Please also follow the kernel coding style.

>  
> -	status = spi_async_locked(spi, message);
> +        if(master->prepare_transfer_hardware)
> +            status = master->prepare_transfer_hardware(master);
> +        if(status >= 0)
> +            status = master->transfer_one_message(master, message);
> +        if(status >= 0 && master->unprepare_transfer_hardware)
> +            status = master->unprepare_transfer_hardware(master);
>  
>  	if (!bus_locked)
>  		mutex_unlock(&master->bus_lock_mutex);
>  
> -	if (status == 0) {
> -		wait_for_completion(&done);
> -		status = message->status;
> -	}

There's many problems with this - the most critical are that it is broken
for multithreaded use, it's not even trying to do locking so both other
callers and the thread will break, and it's not waiting for the transfer
to complete.

It would be good to do as much as we can inline but doing so needs much
more work than this and needs to consider other aspects of performance -
your patch will also make performance worse in many situations.  My talk
at ELC this year covers a lot of this, the slides should be googleable.

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[RFC 2/4] spidev: Avoid runtime memory allocations

[Jeff Epler]

allocating bounce and k_xfers at every iteration was a source of
excess delays.
---
 drivers/spi/spidev.c | 35 +++++++++++++++++++++++++++++------
 1 file changed, 29 insertions(+), 6 deletions(-)

diff --git a/drivers/spi/spidev.c b/drivers/spi/spidev.c
index 2e0655d..dda7632 100644
--- a/drivers/spi/spidev.c
+++ b/drivers/spi/spidev.c
@@ -85,6 +85,8 @@ struct spidev_data {
 	struct mutex		buf_lock;
 	unsigned		users;
 	u8			*buffer;
+        struct spi_ioc_transfer *bounce;
+        struct spi_transfer     *k_xfers;
 };
 
 static LIST_HEAD(device_list);
@@ -227,9 +229,9 @@ static int spidev_message(struct spidev_data *spidev,
 	int			status = -EFAULT;
 
 	spi_message_init(&msg);
-	k_xfers = kcalloc(n_xfers, sizeof(*k_tmp), GFP_KERNEL);
-	if (k_xfers == NULL)
-		return -ENOMEM;
+        if (n_xfers * sizeof(*k_tmp) > bufsiz)
+                return -ENOMEM;
+	k_xfers = spidev->k_xfers;
 
 	/* Construct spi_message, copying any tx data to bounce buffer.
 	 * We walk the array of user-provided transfers, using each one
@@ -302,7 +304,6 @@ static int spidev_message(struct spidev_data *spidev,
 	status = total;
 
 done:
-	kfree(k_xfers);
 	return status;
 }
 
@@ -451,8 +452,12 @@ spidev_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 		if (n_ioc == 0)
 			break;
 
+                if(tmp > bufsiz) {
+			retval = -EINVAL;
+			break;
+                }
 		/* copy into scratch area */
-		ioc = kmalloc(tmp, GFP_KERNEL);
+		ioc = spidev->bounce;
 		if (!ioc) {
 			retval = -ENOMEM;
 			break;
@@ -465,7 +470,6 @@ spidev_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 
 		/* translate to spi_message, execute */
 		retval = spidev_message(spidev, ioc, n_ioc);
-		kfree(ioc);
 		break;
 	}
 
@@ -504,6 +508,19 @@ static int spidev_open(struct inode *inode, struct file *filp)
 				dev_dbg(&spidev->spi->dev, "open/ENOMEM\n");
 				status = -ENOMEM;
 			}
+			spidev->bounce = kmalloc(bufsiz, GFP_KERNEL);
+			if (!spidev->bounce) {
+				dev_dbg(&spidev->spi->dev, "open/ENOMEM\n");
+                                kfree(spidev->buffer);
+				status = -ENOMEM;
+			}
+			spidev->k_xfers = kmalloc(bufsiz, GFP_KERNEL);
+			if (!spidev->k_xfers) {
+				dev_dbg(&spidev->spi->dev, "open/ENOMEM\n");
+                                kfree(spidev->buffer);
+                                kfree(spidev->bounce);
+				status = -ENOMEM;
+			}
 		}
 		if (status == 0) {
 			spidev->users++;
@@ -534,6 +551,12 @@ static int spidev_release(struct inode *inode, struct file *filp)
 		kfree(spidev->buffer);
 		spidev->buffer = NULL;
 
+		kfree(spidev->bounce);
+		spidev->bounce = NULL;
+
+		kfree(spidev->k_xfers);
+		spidev->k_xfers = NULL;
+
 		/* ... after we unbound from the underlying device? */
 		spin_lock_irq(&spidev->spi_lock);
 		dofree = (spidev->spi == NULL);
-- 
2.1.0

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[RFC 3/4] spidev: actually use synchronous transfers

[Jeff Epler]

this reduces a source of latency
---
 drivers/spi/spidev.c | 8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/drivers/spi/spidev.c b/drivers/spi/spidev.c
index dda7632..6e37c59 100644
--- a/drivers/spi/spidev.c
+++ b/drivers/spi/spidev.c
@@ -120,15 +120,9 @@ spidev_sync(struct spidev_data *spidev, struct spi_message *message)
 	if (spidev->spi == NULL)
 		status = -ESHUTDOWN;
 	else
-		status = spi_async(spidev->spi, message);
+		status = spi_sync(spidev->spi, message);
 	spin_unlock_irq(&spidev->spi_lock);
 
-	if (status == 0) {
-		wait_for_completion(&done);
-		status = message->status;
-		if (status == 0)
-			status = message->actual_length;
-	}
 	return status;
 }
 
-- 
2.1.0

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[RFC 4/4] spidev-s3c64xx: allocate dma channel at startup

[Jeff Epler]

it appears that acquire_dma introduces intolerable delays; the system
is fine with the dma channnels allocated all the time.
---
 drivers/spi/spi-s3c64xx.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/drivers/spi/spi-s3c64xx.c b/drivers/spi/spi-s3c64xx.c
index 92de1b5..e6a46c9 100644
--- a/drivers/spi/spi-s3c64xx.c
+++ b/drivers/spi/spi-s3c64xx.c
@@ -797,10 +797,6 @@ static int s3c64xx_spi_prepare_transfer(struct spi_master *spi)
 {
 	struct s3c64xx_spi_driver_data *sdd = spi_master_get_devdata(spi);
 
-	/* Acquire DMA channels */
-	while (!acquire_dma(sdd))
-		msleep(10);
-
 	pm_runtime_get_sync(&sdd->pdev->dev);
 
 	return 0;
@@ -810,10 +806,6 @@ static int s3c64xx_spi_unprepare_transfer(struct spi_master *spi)
 {
 	struct s3c64xx_spi_driver_data *sdd = spi_master_get_devdata(spi);
 
-	/* Free DMA channels */
-	sdd->ops->release(sdd->rx_dma.ch, &s3c64xx_spi_dma_client);
-	sdd->ops->release(sdd->tx_dma.ch, &s3c64xx_spi_dma_client);
-
 	pm_runtime_put(&sdd->pdev->dev);
 
 	return 0;
@@ -1344,6 +1336,9 @@ static int __init s3c64xx_spi_probe(struct platform_device *pdev)
 		goto err7;
 	}
 
+        // cannot fail(?)
+	acquire_dma(sdd);
+
 	writel(S3C64XX_SPI_INT_RX_OVERRUN_EN | S3C64XX_SPI_INT_RX_UNDERRUN_EN |
 	       S3C64XX_SPI_INT_TX_OVERRUN_EN | S3C64XX_SPI_INT_TX_UNDERRUN_EN,
 	       sdd->regs + S3C64XX_SPI_INT_EN);
@@ -1396,6 +1391,10 @@ static int s3c64xx_spi_remove(struct platform_device *pdev)
 
 	spi_unregister_master(master);
 
+	/* Free DMA channels */
+	sdd->ops->release(sdd->rx_dma.ch, &s3c64xx_spi_dma_client);
+	sdd->ops->release(sdd->tx_dma.ch, &s3c64xx_spi_dma_client);
+
 	writel(0, sdd->regs + S3C64XX_SPI_INT_EN);
 
 	free_irq(platform_get_irq(pdev, 0), sdd);
-- 
2.1.0

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