Improves polling mode of s3c64xx driver

[PATCH v2 2/4] spi: s3c64xx: add cpu_relax in polling loop

Posted by Jaewon Kim 2 years, 8 months ago

Adds cpu_relax() to prevent long busy-wait.
There is busy-wait loop to check data transfer completion in polling mode.

Signed-off-by: Jaewon Kim <jaewon02.kim@samsung.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();
 		status = readl(regs + S3C64XX_SPI_STATUS);
 	} while (RX_FIFO_LVL(status, sdd) < xfer->len && --val);
 
-- 
2.17.1

Re: [PATCH v2 2/4] spi: s3c64xx: add cpu_relax in polling loop

Posted by Krzysztof Kozlowski 2 years, 8 months ago

On 19/04/2023 08:06, Jaewon Kim wrote:
> Adds cpu_relax() to prevent long busy-wait.

How cpu_relax prevents long waiting?

> There is busy-wait loop to check data transfer completion in polling mode.
> 
> Signed-off-by: Jaewon Kim <jaewon02.kim@samsung.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?

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

Best regards,
Krzysztof

Re: [PATCH v2 2/4] spi: s3c64xx: add cpu_relax in polling loop

Posted by Jaewon Kim 2 years, 8 months ago

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@samsung.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

Re: [PATCH v2 2/4] spi: s3c64xx: add cpu_relax in polling loop

Posted by Krzysztof Kozlowski 2 years, 8 months ago

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@samsung.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

Re: [PATCH v2 2/4] spi: s3c64xx: add cpu_relax in polling loop

Posted by Jaewon Kim 2 years, 8 months ago

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@samsung.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