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

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Date:   Fri, 21 Apr 2023 10:45:45 +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. 21. 00:39, Krzysztof Kozlowski wrote:
> On 19/04/2023 13:13, Jaewon Kim wrote:
>> 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.
> That's ok, you just wrote that it will prevent long waiting, so I assume
> it will shorten the wait time.
>
>> 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().
> readl_poll_timeout() will know to do the best. You do not need to add
> cpu_relax there.
Okay, I will change it to readl_poll_timeout()
>
> Best regards,
> Krzysztof
>
>

Thanks

Jaewon Kim