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

[Krzysztof Kozlowski]

On Sat, Aug 22, 2020 at 04:52:40PM +0200, Tomasz Figa wrote:
> On Sat, Aug 22, 2020 at 2:43 PM Krzysztof Kozlowski <krzk at 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.

Unfortunately onlt that point of timeout is mentioned in commit msg. If
the correction of timeout was not the point of the patch, then describe
the real point...

> The rate directly determines the SPI transfer
> speed and thus the driver needs to use the rate that was actually set
> for further calculations.

Yep, makes sense.

> 
> > 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).

It is not only this problem. The meaning of cur_speed is now changed.
For !cmu_case this the requested in trasnfer SPI bus clock frequency,
for cmu_case this is now real src_clk frequency. It's getting slightly
messier.

> 
> >
> > 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.

Wrong is here describing one thing and doing much more.

Best regards,
Krzysztof