linux-kernel - Re: [PATCH v2 3/4] spi: s3c64xx: add sleep during transfer

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Date:   Wed, 19 Apr 2023 10:19:54 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Jaewon Kim <jaewon02.kim@...sung.com>,
        Mark Brown <broonie@...nel.org>, Andi Shyti <andi@...zian.org>,
        Alim Akhtar <alim.akhtar@...sung.com>
Cc:     linux-spi@...r.kernel.org, linux-samsung-soc@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Chanho Park <chanho61.park@...sung.com>
Subject: Re: [PATCH v2 3/4] spi: s3c64xx: add sleep during transfer

On 19/04/2023 08:06, Jaewon Kim wrote:
> In polling mode, the status register is constantly read to check transfer
> completion. It cause excessive CPU usage.
> So, it calculates the SPI transfer time and made it sleep.
> 
> Signed-off-by: Jaewon Kim <jaewon02.kim@...sung.com>
> ---
>  drivers/spi/spi-s3c64xx.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/spi/spi-s3c64xx.c b/drivers/spi/spi-s3c64xx.c
> index 886722fb40ea..cf3060b2639b 100644
> --- a/drivers/spi/spi-s3c64xx.c
> +++ b/drivers/spi/spi-s3c64xx.c
> @@ -561,6 +561,14 @@ static int s3c64xx_wait_for_pio(struct s3c64xx_spi_driver_data *sdd,
>  	u32 cpy_len;
>  	u8 *buf;
>  	int ms;
> +	u32 tx_time;
> +
> +	/* sleep during signal transfer time */
> +	status = readl(regs + S3C64XX_SPI_STATUS);
> +	if (RX_FIFO_LVL(status, sdd) < xfer->len) {
> +		tx_time = (xfer->len * 8 * 1000 * 1000) / sdd->cur_speed;
> +		usleep_range(tx_time / 2, tx_time);
> +	}

Did you actually check the delays introduced by it? Is it worth?

>  
>  	/* millisecs to xfer 'len' bytes @ 'cur_speed' */
>  	ms = xfer->len * 8 * 1000 / sdd->cur_speed;

You have now some code duplication so this could be combined.

Best regards,
Krzysztof