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

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Date:   Thu, 20 Apr 2023 17:39:06 +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 2/4] spi: s3c64xx: add cpu_relax in polling loop

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.


Best regards,
Krzysztof