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SubjectRe: [PATCH 3/3] spi: spi-geni-qcom: Slightly optimize setup of bidirectional xfters
On Sat 12 Sep 16:08 CDT 2020, Douglas Anderson wrote:

> When setting up a bidirectional transfer we need to program both the
> TX and RX lengths. We don't need a memory barrier between those two
> writes. Factor out the __iowmb() and use writel_relaxed(). This
> saves a fraction of a microsecond of setup overhead on bidirectional
> transfers.
>
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> ---
>
> drivers/spi/spi-geni-qcom.c | 13 ++++++++++---
> 1 file changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/spi/spi-geni-qcom.c b/drivers/spi/spi-geni-qcom.c
> index 92d88bf85a90..6c7e12b68bf0 100644
> --- a/drivers/spi/spi-geni-qcom.c
> +++ b/drivers/spi/spi-geni-qcom.c
> @@ -376,15 +376,22 @@ static void setup_fifo_xfer(struct spi_transfer *xfer,
> len &= TRANS_LEN_MSK;
>
> mas->cur_xfer = xfer;
> +
> + /*
> + * Factor out the __iowmb() so that we can use writel_relaxed() for
> + * both writes below and thus only incur the overhead once even if
> + * we execute both of them.
> + */

How many passes through this function do we have to take before saving
the amount of time it took me to read this comment?

Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>

Regards,
Bjorn

> + __iowmb();
> +
> if (xfer->tx_buf) {
> m_cmd |= SPI_TX_ONLY;
> mas->tx_rem_bytes = xfer->len;
> - writel(len, se->base + SE_SPI_TX_TRANS_LEN);
> + writel_relaxed(len, se->base + SE_SPI_TX_TRANS_LEN);
> }
> -
> if (xfer->rx_buf) {
> m_cmd |= SPI_RX_ONLY;
> - writel(len, se->base + SE_SPI_RX_TRANS_LEN);
> + writel_relaxed(len, se->base + SE_SPI_RX_TRANS_LEN);
> mas->rx_rem_bytes = xfer->len;
> }
>
> --
> 2.28.0.618.gf4bc123cb7-goog
>

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