linux-kernel - Re: [PATCH v2 7/9] spi: spi-s3c64xx: Ensure cur

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Date:   Mon, 24 Aug 2020 15:21:34 +0200
From:   Tomasz Figa <tfiga@...omium.org>
To:     Lukasz Stelmach <l.stelmach@...sung.com>
Cc:     Krzysztof Kozlowski <krzk@...nel.org>,
        Kukjin Kim <kgene@...nel.org>, Andi Shyti <andi@...zian.org>,
        Mark Brown <broonie@...nel.org>, linux-spi@...r.kernel.org,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        "list@....net:IOMMU DRIVERS <iommu@...ts.linux-foundation.org>, Joerg
        Roedel <joro@...tes.org>," <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Subject: Re: [PATCH v2 7/9] spi: spi-s3c64xx: Ensure cur_speed holds actual
 clock value

On Mon, Aug 24, 2020 at 3:17 PM Lukasz Stelmach <l.stelmach@...sung.com> wrote:
>
> It was <2020-08-22 sob 14:43>, when Krzysztof Kozlowski wrote:
> > On Fri, Aug 21, 2020 at 06:13:59PM +0200, Łukasz Stelmach wrote:
> >> cur_speed is used to calculate transfer timeout and needs to be
> >> set to the actual value of (half) the clock speed for precise
> >> calculations.
> >
> > If you need this only for timeout calculation just divide it in
> > s3c64xx_wait_for_dma().
>
> I divide it here to keep the relationship between the value the variable
> holds and the one that is inside clk_* (See? It's multiplied 3 lines
> above). If you look around every single clk_get_rate() call in the file is
> divided by two.
>
> > Otherwise why only if (cmu) case is updated?
>
> You are righ I will update that too.
>
> However, I wonder if it is even possible that the value read from
> S3C64XX_SPI_CLK_CFG would be different than the one written to it?
>

It is not possible for the register itself, but please see my other
reply, where I explained the integer rounding error which can happen
when calculating the value to write to the register.

> > You are also affecting here not only timeout but
> > s3c64xx_enable_datapath() which is not mentioned in commit log. In other
> > words, this looks wrong.
>
> Indeed, there is a reference too. I've corrected the message.
>

Thanks!

Best regards,
Tomasz

> >>
> >> Cc: Tomasz Figa <tfiga@...omium.org>
> >> Signed-off-by: Łukasz Stelmach <l.stelmach@...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 02de734b8ab1..89c162efe355 100644
> >> --- a/drivers/spi/spi-s3c64xx.c
> >> +++ b/drivers/spi/spi-s3c64xx.c
> >> @@ -626,6 +626,7 @@ static int s3c64xx_spi_config(struct s3c64xx_spi_driver_data *sdd)
> >>              ret = clk_set_rate(sdd->src_clk, sdd->cur_speed * 2);
> >>              if (ret)
> >>                      return ret;
> >> +            sdd->cur_speed = clk_get_rate(sdd->src_clk) / 2;
> >>      } else {
> >>              /* Configure Clock */
> >>              val = readl(regs + S3C64XX_SPI_CLK_CFG);
> >> --
> >> 2.26.2
> >>
> >
> >
>
> --
> Łukasz Stelmach
> Samsung R&D Institute Poland
> Samsung Electronics