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

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Date:   Wed, 19 Apr 2023 17:56:17 +0200
From:   Andi Shyti <andi.shyti@...nel.org>
To:     Jaewon Kim <jaewon02.kim@...sung.com>
Cc:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Mark Brown <broonie@...nel.org>, Andi Shyti <andi@...zian.org>,
        Alim Akhtar <alim.akhtar@...sung.com>,
        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

Hi Jaewon,

> >> 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?
> 
> Yes, I already test it.
> 
> Throughput was the same, CPU utilization decreased to 30~40% from 100%.
> 
> Tested board is ExynosAutov9 SADK.
> 
> 
> >
> >>   
> >>   	/* 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.

you could put the 'if' under the 'ms = ...' and just use ms
without declaring any tx_time.

Andi

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