[PATCH 3/3] spi: bcm2835: add module parameter to configure minimum length for dma

[kernel@martin.sperl.org]

From: Martin Sperl <kernel@martin.sperl.org>

Allow setting the length of the transfer at which dma is used by
setting a module parameter.

Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
---
 drivers/spi/spi-bcm2835.c | 40 +++++++++++++++++++++++++++++++++++++---
 1 file changed, 37 insertions(+), 3 deletions(-)

diff --git a/drivers/spi/spi-bcm2835.c b/drivers/spi/spi-bcm2835.c
index dcf922ca2603..ef4b3468a97d 100644
--- a/drivers/spi/spi-bcm2835.c
+++ b/drivers/spi/spi-bcm2835.c
@@ -31,6 +31,7 @@
 #include <linux/io.h>
 #include <linux/kernel.h>
 #include <linux/module.h>
+#include <linux/moduleparam.h>
 #include <linux/of.h>
 #include <linux/of_address.h>
 #include <linux/of_device.h>
@@ -44,6 +45,39 @@ module_param(polling_limit_us, uint, 0664);
 MODULE_PARM_DESC(polling_limit_us,
 		 "time in us to run a transfer in polling mode\n");

+/* define dma min number of bytes to use in dma mode with value validation */
+static int dma_min_bytes_limit_set(const char *val,
+				   const struct kernel_param *kp)
+{
+	unsigned int v;
+
+	if (kstrtouint(val, 10, &v))
+		return -EINVAL;
+	/* value needs to be a multiple of 4 */
+	if (v % 4) {
+		pr_err("dma_min_bytes_limit needs to be a multiple of 4\n");
+		return -EINVAL;
+	}
+	/* value needs to be at least 6 - so actually 8 - rational below */
+	if (v < 6) {
+		pr_err("dma_min_bytes_limit needs to be at least 8\n");
+		return -EINVAL;
+	}
+
+	return param_set_uint(val, kp);
+}
+
+static const struct kernel_param_ops dma_min_bytes_limit_ops = {
+	.set	= dma_min_bytes_limit_set,
+	.get	= param_get_int,
+};
+
+unsigned int dma_min_bytes_limit = 96;
+module_param_cb(dma_min_bytes_limit, &dma_min_bytes_limit_ops,
+		&dma_min_bytes_limit, 0664);
+MODULE_PARM_DESC(dma_min_bytes_limit,
+		 "minimum number of bytes to run a transfer in dma mode\n");
+
 /* SPI register offsets */
 #define BCM2835_SPI_CS			0x00
 #define BCM2835_SPI_FIFO		0x04
@@ -80,7 +114,6 @@ MODULE_PARM_DESC(polling_limit_us,

 #define BCM2835_SPI_FIFO_SIZE		64
 #define BCM2835_SPI_FIFO_SIZE_3_4	48
-#define BCM2835_SPI_DMA_MIN_LENGTH	96
 #define BCM2835_SPI_MODE_BITS	(SPI_CPOL | SPI_CPHA | SPI_CS_HIGH \
 				| SPI_NO_CS | SPI_3WIRE)

@@ -447,7 +480,8 @@ static int bcm2835_spi_transfer_one_irq(struct spi_master *master,
  * if the length of the first is *exactly* 1.
  *
  * At most 6 bytes are written and at most 3 bytes read.  Do we know the
- * transfer has this many bytes?  Yes, see BCM2835_SPI_DMA_MIN_LENGTH.
+ * transfer has this many bytes?  Yes, see validation in
+ * dma_min_bytes_limit_set.
  *
  * The FIFO is normally accessed with 8-bit width by the CPU and 32-bit width
  * by the DMA engine.  Toggling the DMA Enable flag in the CS register switches
@@ -690,7 +724,7 @@ static bool bcm2835_spi_can_dma(struct spi_master *master,
 				struct spi_transfer *tfr)
 {
 	/* we start DMA efforts only on bigger transfers */
-	if (tfr->len < BCM2835_SPI_DMA_MIN_LENGTH)
+	if (tfr->len < dma_min_bytes_limit)
 		return false;

 	/* BCM2835_SPI_DLEN has defined a max transfer size as
--
2.11.0

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