linux-kernel - Re: [alsa-devel][PATCH] ASoC: wm8960: update pll and clock setting function

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Message-ID: <20150701100615.GL6321@opensource.wolfsonmicro.com>
Date:	Wed, 1 Jul 2015 11:06:15 +0100
From:	Charles Keepax <ckeepax@...nsource.wolfsonmicro.com>
To:	Zidan Wang <zidan.wang@...escale.com>
Cc:	broonie@...nel.org, perex@...ex.cz, tiwai@...e.de, lars@...afoo.de,
	patches@...nsource.wolfsonmicro.com, alsa-devel@...a-project.org,
	linux-kernel@...r.kernel.org
Subject: Re: [alsa-devel][PATCH] ASoC: wm8960: update pll and clock setting
	function

On Wed, Jul 01, 2015 at 04:07:25PM +0800, Zidan Wang wrote:
> On Tue, Jun 30, 2015 at 11:42:00AM +0100, Charles Keepax wrote:
> > On Tue, Jun 30, 2015 at 04:54:09PM +0800, Zidan Wang wrote:
> > > On Mon, Jun 29, 2015 at 10:44:12AM +0100, Charles Keepax wrote:
> > > > On Fri, Jun 26, 2015 at 07:09:22PM +0800, Zidan Wang wrote:
> > > > > When using snd_soc_dai_set_pll to set pll in machine driver, we
> > > > > should set pll in and pll out freq and ensure 5 < PLLN < 13,
> > > > > otherwise set pll will be failed. In order to support more
> > > > > formats and sample rates for a certain MCLK, if snd_soc_dai_set_pll
> > > > > failed, it will calculate a available pll out freq and set the pll
> > > > > again.
> > > > > 
> > > > > Signed-off-by: Zidan Wang <zidan.wang@...escale.com>
> > > > > ---
> > > > 
> > > > I think this need a little more explaination on how this is
> > > > expected to work. From looking at the code what it looks like
> > > > what happens is you can set a PLL frequency through set_pll but
> > > > then if that frequency doesn't support the sample rate requested
> > > > through hw_params it will be changed. This makes me a little
> > > > nervous, as something explicitly requested is being overwritten
> > > > automatically.
> > > > 
> > > From RM, we should ensure 5 < PLLN < 13. When i using snd_soc_dai_set_pll
> > > to set pll frequency, it's hard for me to get a common pll out frequency.
> > > Sometimes, when codec MCLK or sample rate changed, the pll out frequency
> > > also should be changed, otherwise set_pll function will be failed.
> > > 
> > > I made it to auto select pll frequency when snd_soc_dai_set_pll failed, so that
> > > it can support more sample rate, and don't need to set different pll out
> > > frequency for different sample rate and different MCLK.
> > 
> > But none the less I still asked for one frequency and got another
> > frequency, that seems troubling. For example the chip has the
> > ability to output the PLL output on GPIO1, what if that is being
> > used to feed another chip that expects a certain rate?
> > 
> > What are your feeling on my previous suggestion of adding a
> > particular define that indicates set the PLL to "auto" mode?
> Yes, you are right, overwritten the setting is not a good idea. 
> I will change code to support "auto" mode. When it's "auto" mode, if the MCLK can
> provide sysclk, using MCLK directly, otherwise, auto select pll frequency and set PLL.
> 
> Do you think it make sense?

Yeah that sounds reasonable to me.

Thanks,
Charles
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