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

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Date:   Wed, 19 Apr 2023 18:41:05 +0900
From:   Jaewon Kim <jaewon02.kim@...sung.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        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 23. 4. 19. 17:19, Krzysztof Kozlowski wrote:
> 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?

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.
>
> Best regards,
> Krzysztof
>
>
Thanks

Jaewon Kim