linux-kernel - Re: [PATCH v2 2/4] spi: s3c64xx: add cpu

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Date:   Wed, 19 Apr 2023 20:13:28 +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 2/4] spi: s3c64xx: add cpu_relax in polling loop


On 23. 4. 19. 17:14, Krzysztof Kozlowski wrote:
> On 19/04/2023 08:06, Jaewon Kim wrote:
>> Adds cpu_relax() to prevent long busy-wait.
> How cpu_relax prevents long waiting?

As I know, cpu_relax() can be converted to yield. This can prevent 
excessive use of the CPU in busy-loop.

I'll replace poor sentence like below in v3.

("Adds cpu_relax() to allow CPU relaxation in busy-loop")

>> There is busy-wait loop to check data transfer completion in polling mode.
>>
>> Signed-off-by: Jaewon Kim<jaewon02.kim@...sung.com>
>> ---
>>   drivers/spi/spi-s3c64xx.c | 1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/drivers/spi/spi-s3c64xx.c b/drivers/spi/spi-s3c64xx.c
>> index 273aa02322d9..886722fb40ea 100644
>> --- a/drivers/spi/spi-s3c64xx.c
>> +++ b/drivers/spi/spi-s3c64xx.c
>> @@ -568,6 +568,7 @@ static int s3c64xx_wait_for_pio(struct s3c64xx_spi_driver_data *sdd,
>>   
>>   	val = msecs_to_loops(ms);
>>   	do {
>> +		cpu_relax();
> Shouldn't this be just readl_poll_timeout()? Or the syntax would be too
> complicated?

I think we can replace this while() loop to readl_poll_timeout().

However, we should use 0 value as 'delay_us' parameter. Because delay 
can affect throughput.


My purpose is add relax to this busy-loop.

we cannot give relax if we change to readl_poll_timeout().


>>   		status = readl(regs + S3C64XX_SPI_STATUS);
>>   	} while (RX_FIFO_LVL(status, sdd) < xfer->len && --val);
>>   
> Best regards,
> Krzysztof
>
>
Thanks

Jaewon Kim