Re: [PATCH 3/4] clk: bcm2835: De-assert/assert PLL reset signal when appropriate

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

On Thu, 08 Feb 2018 15:15:42 +0000
Eric Anholt <eric@anholt.net> wrote:

> Boris Brezillon <boris.brezillon@bootlin.com> writes:
> 
> > In order to enable a PLL, not only the PLL has to be powered up and
> > locked, but you also have to de-assert the reset signal. The last part
> > was missing. Add it so PLLs that were not enabled by the FW/bootloader
> > can be enabled from Linux.
> >
> > Fixes: 41691b8862e2 ("clk: bcm2835: Add support for programming the audio domain clocks")
> > Cc: <stable@vger.kernel.org>
> > Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
> > ---
> >  drivers/clk/bcm/clk-bcm2835.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> >
> > diff --git a/drivers/clk/bcm/clk-bcm2835.c b/drivers/clk/bcm/clk-bcm2835.c
> > index a07f6451694a..6c5d4a8e426c 100644
> > --- a/drivers/clk/bcm/clk-bcm2835.c
> > +++ b/drivers/clk/bcm/clk-bcm2835.c
> > @@ -602,6 +602,9 @@ static void bcm2835_pll_off(struct clk_hw *hw)
> >  	const struct bcm2835_pll_data *data = pll->data;
> >  
> >  	spin_lock(&cprman->regs_lock);
> > +	cprman_write(cprman, data->a2w_ctrl_reg,
> > +		     cprman_read(cprman, data->a2w_ctrl_reg) &
> > +		     ~A2W_PLL_CTRL_PRST_DISABLE);
> >  	cprman_write(cprman, data->cm_ctrl_reg,
> >  		     cprman_read(cprman, data->cm_ctrl_reg) |
> >  		     CM_PLL_ANARST);  
> 
> For turning off, the FW just does the equivalent of:
> 
> 	cprman_write(cprman, data->cm_ctrl_reg, CM_PLL_ANARST);
> 	cprman_write(cprman, data->a2w_ctrl_reg, A2W_PLL_CTRL_PWRDN);

Hm, the write to ->a2w_ctrl_reg overwrites the 
NDIV/PDIV values done in bcm2835_pll_set_rate(). So, either we do:

	cprman_write(cprman, data->cm_ctrl_reg, CM_PLL_ANARST);
	cprman_write(cprman, data->a2w_ctrl_reg,
		     cprman_read(cprman, data->a2w_ctrl_reg) |
		     A2W_PLL_CTRL_PWRDN);

or we cache the pdiv/ndiv values in struct bcm2835_pll and only apply
them in bcm2835_pll_on().

I'd recommend going for the former to keep the changes easily
backportable to older kernels.

> 
> How about we do that, instead?
> 
> > @@ -640,6 +643,10 @@ static int bcm2835_pll_on(struct clk_hw *hw)
> >  		cpu_relax();
> >  	}
> >  
> > +	cprman_write(cprman, data->a2w_ctrl_reg,
> > +		     cprman_read(cprman, data->a2w_ctrl_reg) |
> > +		     A2W_PLL_CTRL_PRST_DISABLE);
> > +
> >  	return 0;
> >  }  
> 
> I agree with this hunk -- they drop PRST at the very end, after lock.



-- 
Boris Brezillon, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
http://bootlin.com