Re: [PATCH 1/2] spi: Add Renesas R-Car RPC SPI controller driver

[Boris Brezillon <boris.brezillon@bootlin.com>]

On Tue, 20 Nov 2018 09:01:29 +0100
Geert Uytterhoeven <geert@linux-m68k.org> wrote:

> > --- /dev/null
> > +++ b/drivers/spi/spi-renesas-rpc.c
> > @@ -0,0 +1,750 @@  
> 
> > +static int rpc_spi_set_freq(struct rpc_spi *rpc, unsigned long freq)
> > +{
> > +       int ret;
> > +
> > +       if (rpc->cur_speed_hz == freq)
> > +               return 0;
> > +
> > +       clk_disable_unprepare(rpc->clk_rpc);
> > +       ret = clk_set_rate(rpc->clk_rpc, freq);
> > +       if (ret)
> > +               return ret;
> > +
> > +       ret = clk_prepare_enable(rpc->clk_rpc);
> > +       if (ret)
> > +               return ret;  
> 
> The clk_{disable_unprepare,prepare_enable}() may be needed on the Macronix
> controller you based this driver on, but will be futile on Renesas SoCs.
> 
> As the RPC is part of the CPG/MSSR clock domain, its clock will be controlled
> by the Runtime PM.  As you've already called pm_runtime_get_sync() from your
> .probe() calback, Runtime PM will have enabled the clock.
> If you disable it manually, you create an imbalance between automatic and
> manual clock control.
> 
> So please don't control the clock explicitly, but always use
> pm_runtime_*() calls.

More about that. The reason we did that on MXIC is that the clk rate
can't be changed when the clk is enabled. So we have to

1/ explicitly disable the clk that has been enabled by runtime PM
2/ set the new rate
3/ re-enable the clk

So the clk enable/disable are not unbalanced, but it's also true that
this disable/set_rate/enable dance might be unneeded on your platform.