On Mon, Dec 28, 2015 at 05:22:35PM +0100, Marcus Weseloh wrote: > Hi again, > > 2015-12-28 0:29 GMT+01:00 Marcus Weseloh <[email protected]>: > > 2015-12-27 22:09 GMT+01:00 Maxime Ripard <[email protected]>: > [...] > > [...] > >>> - /* Ensure that we have a parent clock fast enough */ > >>> + /* > >>> + * Ensure that the parent clock is set to twice the max speed > >>> + * of the spi device (possibly rounded up by the clk driver) > >>> + */ > >>> mclk_rate = clk_get_rate(sspi->mclk); > >>> - if (mclk_rate < (2 * tfr->speed_hz)) { > >>> - clk_set_rate(sspi->mclk, 2 * tfr->speed_hz); > >>> + if (spi->max_speed_hz != sspi->cur_max_speed || > >>> + mclk_rate != sspi->cur_mclk) { > >> > >> Do you need to cache the values? As far as I'm aware, you end up doing > >> the computation all the time anyway. > > > > By caching the values we optimize the case when a single SPI slave > > device (or even multiple slave devices with the same max_speed) are > > used multiple times in a row. In that case, we're saving two calls: > > clk_set_rate and clk_get_rate. I was unsure about how expensive the > > clk_* calls were, so I thought it would be safer use caching. But > > maybe it's not worth the extra code? > > > > Oh, and I just noticed a mistake in the comment: the clock driver > > rounds up _or_ down, so I should drop the "up". > > As I'm looking further into this, I think the comment should read > "possibly rounded down", as the clk framework is expected to set a > frequency that is less or equal to the requested frequency.
AFAIK, there's no such expectation in the clock framework. You
treating this from a maximum frequency perspective, but it would be
the exact opposite if you were talking about a minimum frequency, as
might be the case for other consumers.
> So the effect I was seeing (that I got a frequency higher than the
> requested one) is actually a bug!? Further details below.
>
> > [...]
> >>> - div = mclk_rate / (2 * tfr->speed_hz);
> >>> - if (div <= (SUN4I_CLK_CTL_CDR2_MASK + 1)) {
> >>> - if (div > 0)
> >>> - div--;
> >>> -
> >>> + div = DIV_ROUND_UP(mclk_rate, 2 * tfr->speed_hz) - 1;
> >>
> >> Isn't it exactly the same thing as mclk_rate / (2 * tfr->speed_hz) ?
> >
> > It is quite often, but not in all cases. The plain division rounds to
> > the nearest integer, so it rounds down sometimes. Consider the
> > following case: we have a slow SPI device with a spi-max-frequency of
> > 50kHz. Our clock driver can't find a clock as slow as 100kHz, so it
> > sets mclk to 214,285Hz.
>
> That clk_set_rate sets a higher frequency than requested seems to be a
> problem in itself. I had a look at clk/sunxi/clk-mod0.c and noticed a
> few small problems there. Will post an RFC patch in a couple of
> minutes.
Did you post these patches already? I think I missed them if that's
the case.
Maxime
--
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com
--
You received this message because you are subscribed to the Google Groups
"linux-sunxi" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
For more options, visit https://groups.google.com/d/optout.
signature.asc
Description: Digital signature
