Re: [alsa-devel] [PATCH v2 2/2] ASoC: codec: wm8960: Relax bit clock computation

[Charles Keepax]

On Tue, Mar 21, 2017 at 04:05:15PM +0200, Daniel Baluta wrote:
> On Tue, Mar 21, 2017 at 2:52 PM, Charles Keepax
> <ckeepax@xxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> > On Tue, Mar 21, 2017 at 12:09:36PM +0200, Daniel Baluta wrote:
> >> WM8960 derives bit clock from sysclock using BCLKDIV[3:0] of R8
> >> clocking register (See WM8960 datasheet, page 71).
> >>
> >> There are use cases, like this:
> >> aplay -Dhw:0,0 -r 48000 -c 1 -f S20_3LE -t raw audio48k20b_3LE1c.pcm
> >>
> >> where no BCLKDIV applied to sysclock can give us the exact requested
> >> bitclk, so driver fails to configure clocking and aplay fails to run.
> >>
> >> Fix this by relaxing bitclk computation, so that when no exact value
> >> can be derived from sysclk pick the closest value greater than
> >> expected bitclk.
> >>
> >> Suggested-by: Charles Keepax <ckeepax@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
> >> Signed-off-by: Daniel Baluta <daniel.baluta@xxxxxxx>
> >> ---
> >> Changes since v1:
> >>       * use a marker to check if a match is found
> >>       * didn't removed PLL as Charles suggested because there is
> >>       a special PLL mode which explictly uses PLL. We could start
> >>       a discussion on not using PLL when deriving bitclk, but this
> >>       is to be done in another patch.
> >>
> >
> > Could you elaborate on this a little more am I not sure I follow
> > 100%? There is a mode which explictly requires the PLL to be used
> > (WM8960_SYSCLK_PLL) but in that case your wm8960_configure_sysclk
> > code will not be called so I don't see what is causing that to have
> > an effect on this patch?
> 
> My doubt is, what happens if wm8960_configure_clocking is called with
> wm8960->clk_id = WM8960_SYSCLK_PLL and we remove the PLL
> as suggested.

I wasn't suggesting removing the PLL just that if we find a
"relaxed match" we don't need to then check the PLL for a better
match, as I suspect that a slightly higher than needed bit clock
has less power/performance impact than firing up the PLL.

Which removes the need to differenciate between a relaxed and
bang on match in wm8960_configure_sysclk and means you don't have
to do the caching the values across the PLL code that you do now.

Thanks,
Charles