linux-kernel - Re: [PATCH V1] ASoC: fsl_ssi: refine ipg clock usage in this module

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Date:	Wed, 10 Sep 2014 10:42:04 -0700
From:	Nicolin Chen <nicoleotsuka@...il.com>
To:	Shengjiu Wang <shengjiu.wang@...escale.com>
Cc:	timur@...i.org, Li.Xiubo@...escale.com, lgirdwood@...il.com,
	broonie@...nel.org, perex@...ex.cz, tiwai@...e.de,
	alsa-devel@...a-project.org, linuxppc-dev@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org, mpa@...gutronix.de
Subject: Re: [PATCH V1] ASoC: fsl_ssi: refine ipg clock usage in this module

On Wed, Sep 10, 2014 at 04:12:53PM +0800, Shengjiu Wang wrote:
> > Then we can get a patch like:
> > open() {
> > +	clk_prepare_enable();
> > 	....
> > }
> > 
> > close() {
> > 	....
> > +	clk_disable_unprepare()
> > }

> what is the open() and close()? do you mean the fsl_ssi_startup()
> and fsl_ssi_shutdown()?

Yea.

> > probe() {
> > 	clk_get();
> > 	clk_prepare_enable();
> > 	....
> > 	if (xxx)
> > -		goto err_xx;
> > +		return ret;
> > 	....
> > +	clk_disable_unprepare();
> > 	return 0;
> > -err_xx:
> > -	clk_disable_unprepare()
> > }

> If this probe() is fsl_ssi_imx_probe(), I think no need to add
> clk_prepare_enable() or clk_disable_unprepare(), seems there is no 
> registers accessing in this probe.

This is trying to be safe, especially for such a driver being used
by multiple platforms. You can omit this as long as the patch can
pass the test on old imx, PowerPC, and AC97 platforms.

>

And another risk just came to my mind is that there would be a
possibility that a machine driver would call set_dai_fmt() early,
after SSI's probe() and before SSI's startup(), if the machine
driver contains dai_fmt assignment in its probe(). Then, without
regmap_mmio_clk(), it'll be tough for us over here because we may
also need to add clock enable/disable for set_dai_fmt/set_sysclk(),
even if there might be still tiny risk that we missed something.

Then there could be a selfish approach to circumvent it is to use
regmap_mmio_clk() with "ipg" at the beginning and call regmap_mmio()
without "ipg" if getting a failed return value from regmap_mmio_clk,
and meanwhile to keep the clock always enabled for the regmap_mmio()
case just like what the current driver is doing. This may result
those non-ipg-clk platforms can't benefit from this refinement
unless they update their DT bindings -- use "ipg" for core clock
This might be the safest and simplest way for us, I'm not sure
everyone would be comfortable with this idea though.

Best regards,
Nicolin
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