On Thu, 08 Feb 2018 15:20:16 +0000
Eric Anholt <eric@xxxxxxxxxx> wrote:
> Boris Brezillon <boris.brezillon@xxxxxxxxxxx> 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@xxxxxxxxxxxxxxx>
> > Signed-off-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxx>
> > ---
> > 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?
--
Boris Brezillon, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
http://bootlin.com