[PATCH 5/9] spi: bcm2835aux: make the polling duration limits configurable

[kernel@xxxxxxxxxxxxxxxx]

From: Martin Sperl <kernel@xxxxxxxxxxxxxxxx>

Under some circumstances the default 30 us polling limit is not optimal
and may lead to long delays because we are waiting on the interrupt
handler resulting in poor spi bus utilization.

With this patch we have the possibility to influence this policy
by making this limit configurable via a module parameters

Signed-off-by: Martin Sperl <kernel@xxxxxxxxxxxxxxxx>
---
Note that this patch relies on the effective_speed_hz patch for the
driver to be applied!

---
 drivers/spi/spi-bcm2835aux.c | 23 +++++++++++++----------
 1 file changed, 13 insertions(+), 10 deletions(-)

diff --git a/drivers/spi/spi-bcm2835aux.c b/drivers/spi/spi-bcm2835aux.c
index 0a5f21eb3b99..af668cb6b38b 100644
--- a/drivers/spi/spi-bcm2835aux.c
+++ b/drivers/spi/spi-bcm2835aux.c
@@ -37,6 +37,12 @@
 #include <linux/spi/spi.h>
 #include <linux/spinlock.h>

+/* define polling limits */
+unsigned int polling_limit_us = 30;
+module_param(polling_limit_us, uint, 0664);
+MODULE_PARM_DESC(polling_limit_us,
+		 "time in us to run a transfer in polling mode\n");
+
 /*
  * spi register defines
  *
@@ -89,10 +95,6 @@
 #define BCM2835_AUX_SPI_STAT_BUSY	0x00000040
 #define BCM2835_AUX_SPI_STAT_BITCOUNT	0x0000003F

-/* timeout values */
-#define BCM2835_AUX_SPI_POLLING_LIMIT_US	30
-#define BCM2835_AUX_SPI_POLLING_JIFFIES		2
-
 struct bcm2835aux_spi {
 	void __iomem *regs;
 	struct clk *clk;
@@ -335,8 +337,8 @@ static int bcm2835aux_spi_transfer_one_poll(struct spi_master *master,
 	bcm2835aux_wr(bs, BCM2835_AUX_SPI_CNTL1, bs->cntl[1]);
 	bcm2835aux_wr(bs, BCM2835_AUX_SPI_CNTL0, bs->cntl[0]);

-	/* set the timeout */
-	timeout = jiffies + BCM2835_AUX_SPI_POLLING_JIFFIES;
+	/* set the timeout to at least 2 jiffies */
+	timeout = jiffies + 2 + HZ * polling_limit_us / 1000000;

 	/* loop until finished the transfer */
 	while (bs->rx_len) {
@@ -381,7 +383,7 @@ static int bcm2835aux_spi_transfer_one(struct spi_master *master,
 				       struct spi_transfer *tfr)
 {
 	struct bcm2835aux_spi *bs = spi_master_get_devdata(master);
-	unsigned long spi_hz, clk_hz, speed;
+	unsigned long spi_hz, clk_hz, speed, hz_per_byte, byte_limit;

 	/* calculate the registers to handle
 	 *
@@ -425,14 +427,15 @@ static int bcm2835aux_spi_transfer_one(struct spi_master *master,
 	 * of Hz per byte per polling limit.  E.g., we can transfer 1 byte in
 	 * 30 µs per 300,000 Hz of bus clock.
 	 */
-#define HZ_PER_BYTE ((9 * 1000000) / BCM2835_AUX_SPI_POLLING_LIMIT_US)
+	hz_per_byte = polling_limit_us ? (9 * 1000000) / polling_limit_us : 0;
+	byte_limit = hz_per_byte ? tfr->effective_speed_hz / hz_per_byte : 1;
+
 	/* run in polling mode for short transfers */
-	if (tfr->len < tfr->effective_speed_hz / HZ_PER_BYTE)
+	if (tfr->len < byte_limit)
 		return bcm2835aux_spi_transfer_one_poll(master, spi, tfr);

 	/* run in interrupt mode for all others */
 	return bcm2835aux_spi_transfer_one_irq(master, spi, tfr);
-#undef HZ_PER_BYTE
 }

 static int bcm2835aux_spi_prepare_message(struct spi_master *master,
--
2.11.0