linux-kernel - Re: [PATCH] spi: s3c64xx: Use DMA mode from fifo size

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Message-ID: <63355869-e679-7226-7719-36b62169db7e@samsung.com>
Date: Fri, 29 Mar 2024 14:53:36 +0900
From: Jaewon Kim <jaewon02.kim@...sung.com>
To: Sam Protsenko <semen.protsenko@...aro.org>
Cc: Andi Shyti <andi.shyti@...nel.org>, Mark Brown <broonie@...nel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>, Alim Akhtar
	<alim.akhtar@...sung.com>, linux-spi@...r.kernel.org,
	linux-samsung-soc@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] spi: s3c64xx: Use DMA mode from fifo size

Hi Sam,

Thanks for your review.


On 3/29/24 02:58, Sam Protsenko wrote:
> On Tue, Mar 26, 2024 at 10:35 PM Jaewon Kim<jaewon02.kim@...sung.com>  wrote:
>> The SPI data size is smaller than FIFO, it operates in PIO mode,
> Spelling: "The" -> "If the"

Thanks, I will fix it v2.

>> and if it is larger than FIFO mode, DMA mode is selected.
>>
>> If the data size is the same as the FIFO size, it operates in PIO mode
>> and data is separated into two transfer. In order to prevent,
> Nit: "transfer" -> "transfers", "prevent" -> "prevent it"

Thanks, I will fix it v2.

>> DMA mode must be used from the case of FIFO and data size.
>>
> You probably mean this code (it occurs two times in the driver):
>
>      xfer->len = fifo_len - 1;
>
> Can you please elaborate on why it's done this way? Why can't we just
> do "xfer->len = fifo_len" and use the whole FIFO for the transfer
> instead? I don't understand the necessity to split the transfer into
> two chunks if its size is of FIFO length -- wouldn't it fit into FIFO
> in that case? (I'm pretty sure this change is correct, just want to
> understand how exactly it works).

In IRQ mode(S3C64XX_SPI_MODE_RX_RDY_LVL enable), TxOverrun/RxUnderrun 
irq occurs when FIFO is full.

To avoid FIFO full, it is transmitted in a smaller size than 
fifo_len.(fifo-len - 1)

However, in case of "fifo_len == data size" "fifo_len - 1" byte + "1" 
byte were transmitted separately.

This problem can be solved by starting DMA transmission start size from 
fifo_len.

>> Fixes: 1ee806718d5e ("spi: s3c64xx: support interrupt based pio mode")
> Just wonder if that fixes some throughput regression, or something
> worse (like failed transfers when the transfer size is the same as
> FIFO size)?

It is not a critical issue, but When I look at the actual waveform, it 
seems strange that only the last 1-byte is transmitted separately.

I thought it was "Fixes", but if not, I will remove it.

>> Signed-off-by: Jaewon Kim<jaewon02.kim@...sung.com>
>> ---
>>   drivers/spi/spi-s3c64xx.c | 6 +++---
>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/spi/spi-s3c64xx.c b/drivers/spi/spi-s3c64xx.c
>> index 9fcbe040cb2f..81ed5fddf83e 100644
>> --- a/drivers/spi/spi-s3c64xx.c
>> +++ b/drivers/spi/spi-s3c64xx.c
>> @@ -430,7 +430,7 @@ static bool s3c64xx_spi_can_dma(struct spi_controller *host,
>>          struct s3c64xx_spi_driver_data *sdd = spi_controller_get_devdata(host);
>>
>>          if (sdd->rx_dma.ch && sdd->tx_dma.ch)
>> -               return xfer->len > sdd->fifo_depth;
>> +               return xfer->len >= sdd->fifo_depth;
>>
>>          return false;
>>   }
>> @@ -826,11 +826,11 @@ static int s3c64xx_spi_transfer_one(struct spi_controller *host,
>>                          return status;
>>          }
>>
>> -       if (!is_polling(sdd) && (xfer->len > fifo_len) &&
>> +       if (!is_polling(sdd) && xfer->len >= fifo_len &&
>>              sdd->rx_dma.ch && sdd->tx_dma.ch) {
>>                  use_dma = 1;
>>
> Would be nice to remove this empty line, while at it.
Good, I will remove it also.
>> -       } else if (xfer->len >= fifo_len) {
>> +       } else if (xfer->len > fifo_len) {
> Below in the same function I can see similar code:
>
>              if (target_len >= fifo_len)
>                  xfer->len = fifo_len - 1;
>
> Shouldn't that 'if' condition be fixed too? Or it's ok as it is? (Just
> noticed it by searching, not sure myself, hence asking).

You are correct. This 'if' condition should not have been modified.

>>                  tx_buf = xfer->tx_buf;
>>                  rx_buf = xfer->rx_buf;
>>                  origin_len = xfer->len;
>> --
>> 2.43.2
>>
>>

Thanks

Jaewon Kim