Dear Trent Piepho,
> mxs_spi_setup_transfer() would set the SSP SCK rate every time it was
> called, which is before each transfer. It is uncommon for the SCK rate to
> change between transfers and this causes unnecessary reprogramming of the
> clock registers. Changed to only set the rate when it has changed.
>
> This significantly speeds up short SPI messages, especially messages made
> up of many transfers. On an iMX287, using spidev with messages that
> consist of 511 transfers of 4 bytes each at an SCK of 48 MHz, the
> effective transfer rate more than doubles from about 290 KB/sec to 600
> KB/sec.
>
> Signed-off-by: Trent Piepho <tpie...@gmail.com>
> Cc: Marek Vasut <ma...@denx.de>
> Cc: Fabio Estevam <fabio.este...@freescale.com>
> Cc: Shawn Guo <shawn....@linaro.org>
> ---
> drivers/spi/spi-mxs.c | 15 ++++++++++++++-
> 1 file changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/spi/spi-mxs.c b/drivers/spi/spi-mxs.c
> index a7d5f85..12794ea 100644
> --- a/drivers/spi/spi-mxs.c
> +++ b/drivers/spi/spi-mxs.c
> @@ -68,6 +68,7 @@
> struct mxs_spi {
> struct mxs_ssp ssp;
> struct completion c;
Stick the comment here.
> + unsigned int sck; /* Rate requested (vs actual) */
> };
>
> static int mxs_spi_setup_transfer(struct spi_device *dev,
> @@ -88,7 +89,19 @@ static int mxs_spi_setup_transfer(struct spi_device
> *dev, return -EINVAL;
> }
>
> - mxs_ssp_set_clk_rate(ssp, hz);
> + if (hz != spi->sck) {
> + mxs_ssp_set_clk_rate(ssp, hz);
> + /*
> + * Save requested value, not actual value from ssp->clk_rate.
> + * Otherwise we would set the rate again each trasfer when
> + * actual is not quite the same as requested.
> + */
> + spi->sck = hz;
> + /*
> + * Perhaps we should return an error if the actual clock is
> + * nowhere close to what was requested?
> + */
Maybe you want to check the ssp->ssp_clk vs. hz indeed.
> + }
>
> writel(BM_SSP_CTRL0_LOCK_CS,
> ssp->base + HW_SSP_CTRL0 + STMP_OFFSET_REG_SET);
Best regards,
Marek Vasut
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