linux-kernel - Re: [PATCH 3/3] spi: spi-geni-qcom: Slightly optimize setup of bidirectional xfters

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Message-ID: <20200912225440.GB3715@yoga>
Date:   Sat, 12 Sep 2020 17:54:40 -0500
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     Mark Brown <broonie@...nel.org>,
        Akash Asthana <akashast@...eaurora.org>, swboyd@...omium.org,
        Andy Gross <agross@...nel.org>, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-spi@...r.kernel.org
Subject: Re: [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@...omium.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@...aro.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
>