From: Vladimir Oltean <olteanv@gmail.com>
Date: Tue, 1 Oct 2019 23:52:16 +0300
Subject: spi: spi-fsl-dspi: Always use the TCFQ devices in poll mode
Git-commit: 3c0f9d8bcf47ed33f479cf9dc933d405020aefe4
Patch-mainline: v5.5-rc1
References: bsc#1167260
With this patch, the "interrupts" property from the device tree bindings
is ignored, even if present, if the driver runs in TCFQ mode.
Switching to using the DSPI in poll mode has several distinct
benefits:
- With interrupts, the DSPI driver in TCFQ mode raises an IRQ after each
transmitted word. There is more time wasted for the "waitq" event than
for actual I/O. And the DSPI IRQ count can easily get the largest in
/proc/interrupts on Freescale boards with attached SPI devices.
- The SPI I/O time is both lower, and more consistently so. Attached to
some Freescale devices are either PTP switches, or SPI RTCs. For
reading time off of a SPI slave device, it is important that all SPI
transfers take a deterministic time to complete.
- In poll mode there is much less time spent by the CPU in hardirq
context, which helps with the response latency of the system, and at
the same time there is more control over when interrupts must be
disabled (to get a precise timestamp measurement, which will come in a
future patch): win-win.
On the LS1021A-TSN board, where the SPI device is a SJA1105 PTP switch
(with a bits_per_word=8 driver), I created a "benchmark" where I
periodically transferred a 12-byte message once per second, for 120
seconds. I then recorded the time before putting the first byte in the
TX FIFO, and the time after reading the last byte from the RX FIFO. That
is the transfer delay in nanoseconds.
Interrupt mode:
delay: min 125120 max 168320 mean 150286 std dev 17675.3
Poll mode:
delay: min 69440 max 119040 mean 70312.9 std dev 8065.34
Both the mean latency and the standard deviation are more than 50% lower
in poll mode than in interrupt mode, and the 'max' in poll mode is lower
than the 'min' in interrupt mode. This is with an 'ondemand' governor on
an otherwise idle system - therefore running mostly at 600 MHz out of a
max of 1200 MHz.
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20191001205216.32115-1-olteanv@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Mian Yousaf Kaukab <ykaukab@suse.de>
---
drivers/spi/spi-fsl-dspi.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/drivers/spi/spi-fsl-dspi.c b/drivers/spi/spi-fsl-dspi.c
index 2c0f211eed87..c61074502145 100644
--- a/drivers/spi/spi-fsl-dspi.c
+++ b/drivers/spi/spi-fsl-dspi.c
@@ -707,7 +707,7 @@ static irqreturn_t dspi_interrupt(int irq, void *dev_id)
regmap_read(dspi->regmap, SPI_SR, &spi_sr);
regmap_write(dspi->regmap, SPI_SR, spi_sr);
- if (!(spi_sr & (SPI_SR_EOQF | SPI_SR_TCFQF)))
+ if (!(spi_sr & SPI_SR_EOQF))
return IRQ_NONE;
if (dspi_rxtx(dspi) == 0) {
@@ -1134,6 +1134,9 @@ static int dspi_probe(struct platform_device *pdev)
dspi_init(dspi);
+ if (dspi->devtype_data->trans_mode == DSPI_TCFQ_MODE)
+ goto poll_mode;
+
dspi->irq = platform_get_irq(pdev, 0);
if (dspi->irq <= 0) {
dev_info(&pdev->dev,
--
2.26.2