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

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Message-ID: <CAPLW+4k7GnK+jqbH2t8ZEMwFCW9nVD84k9ca3YY9KfDaiSkOTA@mail.gmail.com>
Date: Fri, 29 Mar 2024 01:01:16 -0500
From: Sam Protsenko <semen.protsenko@...aro.org>
To: Jaewon Kim <jaewon02.kim@...sung.com>
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

On Fri, Mar 29, 2024 at 12:53 AM Jaewon Kim <jaewon02.kim@...sung.com> wrote:
>
> 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.
>

Thanks for the explanation! Please feel free to add:

Reviewed-by: Sam Protsenko <semen.protsenko@...aro.org>

> >> 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.
>

No no, I was just curious. "Fixes" is fine with me.

> >> 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
>