Re: [PATCH v2 7/9] spi: spi-s3c64xx: Ensure cur_speed holds actual clock value

[Tomasz Figa <tfiga@chromium.org>]

On Sat, Aug 22, 2020 at 2:43 PM Krzysztof Kozlowski <krzk@kernel.org> 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().

Division is not the point of the patch. The point is that
clk_set_rate() that was originally there doesn't guarantee that the
rate is set exactly. The rate directly determines the SPI transfer
speed and thus the driver needs to use the rate that was actually set
for further calculations.

> Otherwise why only if (cmu) case is updated?

Right, the !cmu case actually suffers from the same problem. The code
divides the parent clock rate with the requested speed to obtain the
divider to program into the register. This is subject to integer
rounding, so (parent / (parent / speed)) doesn't always equal (speed).

>
> 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.

Actually this is right and fixes one more problem, which I didn't spot
when looking at this code when I suggested the change (I only spotted
the effects on timeout calculation). The rounding error might have
caused wrong configuration there, because e.g. 30000000 Hz could be
requested and rounded to 28000000 Hz. The former is a threshold for
setting the S3C64XX_SPI_CH_HS_EN bit, but the real frequency wouldn't
actually require setting it.

Best regards,
Tomasz

>
> Best regards,
> Krzysztof
>
> >
> > Cc: Tomasz Figa <tfiga@chromium.org>
> > Signed-off-by: Łukasz Stelmach <l.stelmach@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 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
> >