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

[Daniel Baluta <daniel.baluta@gmail.com>]

On Tue, Mar 21, 2017 at 4:20 PM, Charles Keepax
<ckeepax@opensource.wolfsonmicro.com> wrote:
> 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@opensource.wolfsonmicro.com> 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@opensource.wolfsonmicro.com>
>> >> Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
>> >> ---
>> >> 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.

Oh, I see. So we still use the PLL when no exact or relaxed match
is found.