Re: [PATCH 4/4] clk: bcm2835: Make sure the PLL is gated before changing its rate

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

-Stephen Warren
+Stefan Wahren

On Fri, 09 Feb 2018 09:32:40 +0000
Eric Anholt <eric@anholt.net> wrote:

> Boris Brezillon <boris.brezillon@bootlin.com> writes:
> 
> > On Thu, 08 Feb 2018 15:20:16 +0000
> > Eric Anholt <eric@anholt.net> wrote:
> >  
> >> Boris Brezillon <boris.brezillon@bootlin.com> writes:
> >>   
> >> > All bcm2835 PLLs should be gated before their rate can be changed.
> >> > Setting CLK_SET_RATE_GATE will let the core enforce that, but this is
> >> > not enough to make the code work in all situations. Indeed, the
> >> > CLK_SET_RATE_GATE flag prevents a user from changing the rate while
> >> > the clock is enabled, but this check only guarantees there's no Linux
> >> > users. In our case, the clock might have been enabled by the
> >> > bootloader/FW, and, because we have CLK_IGNORE_UNUSED set, Linux never
> >> > disables the PLL. So we have to make sure the PLL is actually disabled
> >> > before changing the rate.
> >> >
> >> > 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 | 14 +++++++++++++-
> >> >  1 file changed, 13 insertions(+), 1 deletion(-)
> >> >
> >> > diff --git a/drivers/clk/bcm/clk-bcm2835.c b/drivers/clk/bcm/clk-bcm2835.c
> >> > index 6c5d4a8e426c..051ce769c109 100644
> >> > --- a/drivers/clk/bcm/clk-bcm2835.c
> >> > +++ b/drivers/clk/bcm/clk-bcm2835.c
> >> > @@ -678,6 +678,18 @@ static int bcm2835_pll_set_rate(struct clk_hw *hw,
> >> >  	u32 ana[4];
> >> >  	int i;
> >> >  
> >> > +	/*
> >> > +	 * Normally, the CLK_SET_RATE_GATE flag prevents a user from changing
> >> > +	 * the rate while the clock is enabled, but this check only makes sure
> >> > +	 * there's no Linux users.
> >> > +	 * In our case, the clock might have been enabled by the bootloader/FW,
> >> > +	 * and, since CLK_IGNORE_UNUSED flag is set, Linux never disables it.
> >> > +	 * So we have to make sure the clk is actually disabled before changing
> >> > +	 * the rate.
> >> > +	 */
> >> > +	if (bcm2835_pll_is_on(hw))
> >> > +		bcm2835_pll_off(hw);
> >> > +    
> >> 
> >> I'm not sure this improves the situation.  If the PLL was on, then
> >> presumably there's a divider using it and a CM clock using that, so
> >> we'll probably end up driving some glitches on them.  
> >
> > Hm, yes, but if someone is trying to change the rate of the PLL, and the
> > core doesn't know other clks depend on this PLL (which is the case if
> > we reach this point), we're already in big trouble.
> >  
> >> 
> >> Does the common clk framework have a way to disable unused clocks from
> >> the leaf clocks up to this root, before the general
> >> disable-unused-clocks path happens late in the boot process?  
> >
> > Not that I know of. What do you have in mind?   
> 
> I was hoping that Stephen Boyd or Mike might have an answer for this
> problem.

Having a generic solution for this sort of issue is definitely the
way to go, but I think this temporary hack is needed to make HDMI/SDTV
work properly. If we don't have it and the FW configures and enables
PLLH with a rate that is different from the one the HDMI or SDTV
encoder tries to set, we're screwed, because I doubt the CPRMAN block
allows you to change the rate of the PLL when it's not gated. Which
means the new rate is not applied and the clk user has no way of
knowing that, which in turn means the display output is likely to not
work properly the first time it's enabled.

Of course, this all goes away the second time the HDMI/SDTV encoder is
enabled, because then clk_disable_unprepare() is called which has the
effect of disabling the PLL.

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