Re: [PATCH v7 4/5] clk: Provide critical clock support

Lee Jones
2015-08-17T07:42:50Z

On Mon, 17 Aug 2015, Barry Song wrote:

> 2015-07-22 21:04 GMT+08:00 Lee Jones <lee.jones@linaro.org>:
> > Lots of platforms contain clocks which if turned off would prove fatal.
> > The only way to recover from these catastrophic failures is to restart
> > the board(s). Now, when a clock provider is registered with the
> > framework it is possible for a list of critical clocks to be supplied
> > which must be kept ungated. Each clock mentioned in the newly
> > introduced 'critical-clock' property will be clk_prepare_enable()d
> > where the normal references will be taken. This will prevent the
> > common clk framework from attempting to gate them during the normal
> > clk_disable_unused() and disable_clock() procedures.
> >
> > Note that it will still be possible for knowledgeable drivers to turn
> > these clocks off using clk_{enable,disable}_critical() calls.
> >
> > Signed-off-by: Lee Jones <lee.jones@linaro.org>
>
> hi Lee,
> i have another email about this. i am wondering whether
> CLK_IGNORE_UNUSE is still useful after your patch. another solution
> for your patch might be extending the current CLK_IGNORE_UNUSE to
> runtime?
>
>
> copy the mail here:
> currently we can set a CLK_IGNORE_UNUSE flag to a clock to stop
> clk_disable_unused() from disabling it at the boot stage:
>
> static void clk_disable_unused_subtree(struct clk_core *core)
> {
> ...
>
> if (core->flags & CLK_IGNORE_UNUSED)
> goto unlock_out;
> }
>
> static int clk_disable_unused(void)
> {
> ...
>
> clk_unprepare_unused_subtree(core);
> ...
> return 0;
> }
>
> late_initcall_sync(clk_disable_unused);
>
> so if there are two clocks A and B, A is the parent of B, and A is
> marked as CLK_IGNORE_UNUSED.
>
> in boot stage if there is nobody using A and B, Linux will disable B
> due to clk_disable_unused() , but keep A being enabled since A has
> CLK_IGNORE_UNUSED.
>
> but in Linux runtime, we might frequently disable /enable B in runtime
> power management, this will cause A disabled since Linux will not
> check CLK_IGNORE_UNUSED for runtime disabling clk .
>
> so this makes CLK_IGNORE_UNUSE only work if we don't disable its
> sub-clock at runtime. this looks making no sense.
>
> i am thinking whether we should do some changes to make it have side
> affect for runtime clk disable. otherwise, why do we want to stop it
> from being disabled during boot stage?

This is one of this problems, along with some others that this set
aims to solve.

Extending CLK_IGNORE_UNUSED is not a good idea. In fact, if we can
phase it out completely, that will be a good thing.

Since this set Mike has submitted an alternitive solution.

Please see: https://groups.google.com/forum/#!msg/linux.kernel/kX_nWSsWRxU/IZSjhG5Ed4oJ

> I am not sure whether i missed something in clk core level support.
>
> -barry
>
> > ---
> > drivers/clk/clk-conf.c | 45 ++++++++++++++++++++++++++++++++++++++++++++-
> > 1 file changed, 44 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/clk/clk-conf.c b/drivers/clk/clk-conf.c
> > index aad4796..f83c1c2 100644
> > --- a/drivers/clk/clk-conf.c
> > +++ b/drivers/clk/clk-conf.c
> > @@ -116,6 +116,45 @@ static int __set_clk_rates(struct device_node *node, bool clk_supplier)
> > return 0;
> > }
> >
> > +static int __set_critical_clocks(struct device_node *node, bool clk_supplier)
> > +{
> > + struct of_phandle_args clkspec;
> > + struct clk *clk;
> > + struct property *prop;
> > + const __be32 *cur;
> > + uint32_t index;
> > + int ret;
> > +
> > + if (!clk_supplier)
> > + return 0;
> > +
> > + of_property_for_each_u32(node, "critical-clock", prop, cur, index) {
> > + clkspec.np = node;
> > + clkspec.args_count = 1;
> > + clkspec.args[0] = index;
> > +
> > + clk = of_clk_get_from_provider(&clkspec);
> > + if (IS_ERR(clk)) {
> > + pr_err("clk: couldn't get clock %u for %s\n",
> > + index, node->full_name);
> > + return PTR_ERR(clk);
> > + }
> > +
> > + clk_init_critical(clk);
> > +
> > + ret = clk_prepare_enable(clk);
> > + if (ret) {
> > + pr_err("Failed to enable clock %u for %s: %d\n",
> > + index, node->full_name, ret);
> > + return ret;
> > + }
> > +
> > + pr_debug("Setting clock as critical %pC\n", clk);
> > + }
> > +
> > + return 0;
> > +}
> > +
> > /**
> > * of_clk_set_defaults() - parse and set assigned clocks configuration
> > * @node: device node to apply clock settings for
> > @@ -139,6 +178,10 @@ int of_clk_set_defaults(struct device_node *node, bool clk_supplier)
> > if (rc < 0)
> > return rc;
> >
> > - return __set_clk_rates(node, clk_supplier);
> > + rc = __set_clk_rates(node, clk_supplier);
> > + if (rc < 0)
> > + return rc;
> > +
> > + return __set_critical_clocks(node, clk_supplier);
> > }
> > EXPORT_SYMBOL_GPL(of_clk_set_defaults);

--
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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