linux-kernel - Re: [PATCH] sound: soc: fsl: Do not set DAI sysclk when it is equal to system freq

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Message-ID: <20170907134411.uzwh54lstzmvp6dd@sirena.co.uk>
Date:   Thu, 7 Sep 2017 14:44:11 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Nicolin Chen <nicoleotsuka@...il.com>
Cc:     Łukasz Majewski <lukma@...x.de>,
        Timur Tabi <timur@...i.org>, Xiubo Li <Xiubo.Lee@...il.com>,
        Fabio Estevam <fabio.estevam@....com>,
        Liam Girdwood <lgirdwood@...il.com>,
        Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>, alsa-devel@...a-project.org,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sound: soc: fsl: Do not set DAI sysclk when it is equal
 to system freq

On Tue, Sep 05, 2017 at 10:45:29AM -0700, Nicolin Chen wrote:

> The ipg clock is merely used to access registers, and has nothing
> (directly) to do with external clock outputs. The driver shall not
> change the ipg clock as the system ipg clock (its parent clock)
> might be messed and even system time would get weird -- happened
> once when the fsl_spdif driver used to call clk_set_rate() on its
> ipg clock. Although the clock controller should have some kind of
> protection in my opinion, we just avoid IP clock rate change in all
> audio drivers as well.

Yes, the clock API needs constraints code.

> On the other hand, the sys clock (baudclk in the driver) should be
> configured whenever it's related to external clock outputs. When I
> implemented this set_sysclk() for fsl_ssi.c, I used it to set this
> sys clock (baudclk) by a machine driver, in order to set bit clock.
> Then someone patched the driver by moving all the code to set_bclk()
> to make machine drivers simpler. Now the set_sysclk() is remained
> to give machine drivers a chance to override clock configurations
> in the hw_params(). This could be used in TDM or some other special
> cases (It could also have a purpose for backwards compatibility).

> So here, we should set baudclk (BCLK generator).

No, that's just going to cause confusion - if all the other drivers are
using set_sysclk() to set an input clock rate to the IP rather than an
output clock but your driver does something else then sooner or later
someone will run into trouble with that.  

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