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.