spi: bcm2835: enabling polling mode for transfers shorter than 30us
In cases of short transfer times the CPU is spending lots of time
in the interrupt handler and scheduler to reschedule the worker thread.
Measurements show that we have times where it takes 29.32us to between
the last clock change and the time that the worker-thread is running again
returning from wait_for_completion_timeout().
During this time the interrupt-handler is running calling complete()
and then also the scheduler is rescheduling the worker thread.
This time can vary depending on how much of the code is still in
CPU-caches, when there is a burst of spi transfers the subsequent delays
are in the order of 25us, so the value of 30us seems reasonable.
With polling the whole transfer of 4 bytes at 10MHz finishes after 6.16us
(CS down to up) with the real transfer (clock running) taking 3.56us.
So the efficiency has much improved and is also freeing CPU cycles,
reducing interrupts and context switches.
Because of the above 30us seems to be a reasonable limit for polling.
Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
diff --git a/drivers/spi/spi-bcm2835.c b/drivers/spi/spi-bcm2835.c
index 9006449..f63864a 100644
--- a/drivers/spi/spi-bcm2835.c
+++ b/drivers/spi/spi-bcm2835.c
@@ -68,7 +68,8 @@
#define BCM2835_SPI_CS_CS_10 0x00000002
#define BCM2835_SPI_CS_CS_01 0x00000001
-#define BCM2835_SPI_TIMEOUT_MS 30000
+#define BCM2835_SPI_POLLING_LIMIT_US 30
+#define BCM2835_SPI_TIMEOUT_MS 30000
#define BCM2835_SPI_MODE_BITS (SPI_CPOL | SPI_CPHA | SPI_CS_HIGH \
| SPI_NO_CS | SPI_3WIRE)
@@ -156,52 +157,49 @@
return IRQ_HANDLED;
}
-static int bcm2835_spi_transfer_one(struct spi_master *master,
- struct spi_device *spi,
- struct spi_transfer *tfr)
+static int bcm2835_spi_transfer_one_poll(struct spi_master *master,
+ struct spi_device *spi,
+ struct spi_transfer *tfr,
+ u32 cs,
+ unsigned long xfer_time_us)
{
struct bcm2835_spi *bs = spi_master_get_devdata(master);
- unsigned long spi_hz, clk_hz, cdiv;
- u32 cs = bcm2835_rd(bs, BCM2835_SPI_CS);
+ unsigned long timeout = jiffies +
+ max(4 * xfer_time_us * HZ / 1000000, 2uL);
- /* set clock */
- spi_hz = tfr->speed_hz;
- clk_hz = clk_get_rate(bs->clk);
+ /* enable HW block without interrupts */
+ bcm2835_wr(bs, BCM2835_SPI_CS, cs | BCM2835_SPI_CS_TA);
- if (spi_hz >= clk_hz / 2) {
- cdiv = 2; /* clk_hz/2 is the fastest we can go */
- } else if (spi_hz) {
- /* CDIV must be a multiple of two */
- cdiv = DIV_ROUND_UP(clk_hz, spi_hz);
- cdiv += (cdiv % 2);
-
- if (cdiv >= 65536)
- cdiv = 0; /* 0 is the slowest we can go */
- } else {
- cdiv = 0; /* 0 is the slowest we can go */
+ /* set timeout to 4x the expected time, or 2 jiffies */
+ /* loop until finished the transfer */
+ while (bs->rx_len) {
+ /* read from fifo as much as possible */
+ bcm2835_rd_fifo(bs);
+ /* fill in tx fifo as much as possible */
+ bcm2835_wr_fifo(bs);
+ /* if we still expect some data after the read,
+ * check for a possible timeout
+ */
+ if (bs->rx_len && time_after(jiffies, timeout)) {
+ /* Transfer complete - reset SPI HW */
+ bcm2835_spi_reset_hw(master);
+ /* and return timeout */
+ return -ETIMEDOUT;
+ }
}
- bcm2835_wr(bs, BCM2835_SPI_CLK, cdiv);
- /* handle all the modes */
- if ((spi->mode & SPI_3WIRE) && (tfr->rx_buf))
- cs |= BCM2835_SPI_CS_REN;
- if (spi->mode & SPI_CPOL)
- cs |= BCM2835_SPI_CS_CPOL;
- if (spi->mode & SPI_CPHA)
- cs |= BCM2835_SPI_CS_CPHA;
+ /* Transfer complete - reset SPI HW */
+ bcm2835_spi_reset_hw(master);
+ /* and return without waiting for completion */
+ return 0;
+}
- /* for gpio_cs set dummy CS so that no HW-CS get changed
- * we can not run this in bcm2835_spi_set_cs, as it does
- * not get called for cs_gpio cases, so we need to do it here
- */
- if (gpio_is_valid(spi->cs_gpio) || (spi->mode & SPI_NO_CS))
- cs |= BCM2835_SPI_CS_CS_10 | BCM2835_SPI_CS_CS_01;
-
- /* set transmit buffers and length */
- bs->tx_buf = tfr->tx_buf;
- bs->rx_buf = tfr->rx_buf;
- bs->tx_len = tfr->len;
- bs->rx_len = tfr->len;
+static int bcm2835_spi_transfer_one_irq(struct spi_master *master,
+ struct spi_device *spi,
+ struct spi_transfer *tfr,
+ u32 cs)
+{
+ struct bcm2835_spi *bs = spi_master_get_devdata(master);
/* fill in fifo if we have gpio-cs
* note that there have been rare events where the native-CS
@@ -232,6 +230,68 @@
return 1;
}
+static int bcm2835_spi_transfer_one(struct spi_master *master,
+ struct spi_device *spi,
+ struct spi_transfer *tfr)
+{
+ struct bcm2835_spi *bs = spi_master_get_devdata(master);
+ unsigned long spi_hz, clk_hz, cdiv;
+ unsigned long spi_used_hz, xfer_time_us;
+ u32 cs = bcm2835_rd(bs, BCM2835_SPI_CS);
+
+ /* set clock */
+ spi_hz = tfr->speed_hz;
+ clk_hz = clk_get_rate(bs->clk);
+
+ if (spi_hz >= clk_hz / 2) {
+ cdiv = 2; /* clk_hz/2 is the fastest we can go */
+ } else if (spi_hz) {
+ /* CDIV must be a multiple of two */
+ cdiv = DIV_ROUND_UP(clk_hz, spi_hz);
+ cdiv += (cdiv % 2);
+
+ if (cdiv >= 65536)
+ cdiv = 0; /* 0 is the slowest we can go */
+ } else {
+ cdiv = 0; /* 0 is the slowest we can go */
+ }
+ spi_used_hz = cdiv ? (clk_hz / cdiv) : (clk_hz / 65536);
+ bcm2835_wr(bs, BCM2835_SPI_CLK, cdiv);
+
+ /* handle all the modes */
+ if ((spi->mode & SPI_3WIRE) && (tfr->rx_buf))
+ cs |= BCM2835_SPI_CS_REN;
+ if (spi->mode & SPI_CPOL)
+ cs |= BCM2835_SPI_CS_CPOL;
+ if (spi->mode & SPI_CPHA)
+ cs |= BCM2835_SPI_CS_CPHA;
+
+ /* for gpio_cs set dummy CS so that no HW-CS get changed
+ * we can not run this in bcm2835_spi_set_cs, as it does
+ * not get called for cs_gpio cases, so we need to do it here
+ */
+ if (gpio_is_valid(spi->cs_gpio) || (spi->mode & SPI_NO_CS))
+ cs |= BCM2835_SPI_CS_CS_10 | BCM2835_SPI_CS_CS_01;
+
+ /* set transmit buffers and length */
+ bs->tx_buf = tfr->tx_buf;
+ bs->rx_buf = tfr->rx_buf;
+ bs->tx_len = tfr->len;
+ bs->rx_len = tfr->len;
+
+ /* calculate the estimated time in us the transfer runs */
+ xfer_time_us = tfr->len
+ * 9 /* clocks/byte - SPI-HW waits 1 clock after each byte */
+ * 1000000 / spi_used_hz;
+
+ /* for short requests run polling*/
+ if (xfer_time_us <= BCM2835_SPI_POLLING_LIMIT_US)
+ return bcm2835_spi_transfer_one_poll(master, spi, tfr,
+ cs, xfer_time_us);
+
+ return bcm2835_spi_transfer_one_irq(master, spi, tfr, cs);
+}
+
static void bcm2835_spi_handle_err(struct spi_master *master,
struct spi_message *msg)
{