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

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Date:   Tue, 21 Mar 2017 16:05:15 +0200
From:   Daniel Baluta <daniel.baluta@...il.com>
To:     Charles Keepax <ckeepax@...nsource.wolfsonmicro.com>
Cc:     Daniel Baluta <daniel.baluta@....com>, alsa-devel@...a-project.org,
        shengjiu.wang@...escale.com, patches@...nsource.wolfsonmicro.com,
        Liam Girdwood <lgirdwood@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>, viorel.suman@....com,
        mihai.serban@....com, Takashi Iwai <tiwai@...e.com>
Subject: Re: [alsa-devel] [PATCH v2 2/2] ASoC: codec: wm8960: Relax bit clock computation

On Tue, Mar 21, 2017 at 2:52 PM, Charles Keepax
<ckeepax@...nsource.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@...nsource.wolfsonmicro.com>
>> Signed-off-by: Daniel Baluta <daniel.baluta@....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.

Is this possible even possible?

Anyhow, I noticed that so far wm8960->clk_id is never set to
WM8960_SYSCLK_PLL :).

So, my proposal is to merge this patch which improves the existing code and deal
with PLL fallback in a separate patch later.

I am afraid of touching things which I don't understand how they work :D.

thanks,
Daniel.