Re: [PATCH RFC RFT 2/3] clk: clk_put WARNs if user has not disabled clk

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Mike, Russell,

On Tue, Oct 20, 2015 at 2:40 PM, Michael Turquette
<mturquette@baylibre.com> wrote:
> Quoting Geert Uytterhoeven (2015-09-30 08:38:46)
>> On Fri, Aug 7, 2015 at 9:09 PM, Michael Turquette
>> <mturquette@baylibre.com> wrote:
>> > From the clk_put kerneldoc in include/linux/clk.h:
>> >
>> > """
>> > Note: drivers must ensure that all clk_enable calls made on this clock
>> > source are balanced by clk_disable calls prior to calling this function.
>> > """
>> >
>> > The common clock framework implementation of the clk.h api has per-user
>> > reference counts for calls to clk_prepare and clk_disable. As such it
>> > can enforce the requirement to properly call clk_disable and
>> > clk_unprepare before calling clk_put.
>> >
>> > Because this requirement is probably violated in many places, this patch
>> > starts with a simple warning. Once offending code has been fixed this
>> > check could additionally release the reference counts automatically.
>> >
>> > Signed-off-by: Michael Turquette <mturquette@baylibre.com>
>> > ---
>> >  drivers/clk/clk.c | 8 ++++++++
>> >  1 file changed, 8 insertions(+)
>> >
>> > diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
>> > index 72feee9..6ec0f77 100644
>> > --- a/drivers/clk/clk.c
>> > +++ b/drivers/clk/clk.c
>> > @@ -2764,6 +2764,14 @@ void __clk_put(struct clk *clk)
>> >             clk->max_rate < clk->core->req_rate)
>> >                 clk_core_set_rate_nolock(clk->core, clk->core->req_rate);
>> >
>> > +       /*
>> > +        * before calling clk_put, all calls to clk_prepare and clk_enable from
>> > +        * a given user must be balanced with calls to clk_disable and
>> > +        * clk_unprepare by that same user
>> > +        */
>> > +       WARN_ON(clk->prepare_count);
>> > +       WARN_ON(clk->enable_count);
>>
>> These two WARN_ON()s are triggered a lot when using a legacy clock domain,
>> and CONFIG_PM=n. Indeed, without Runtime PM, the idea is that the module clocks
>> get enabled unconditionally, which violates the assumptions above.
>>
>> Cfr. the CONFIG_PM=n version of pm_clk_notify() in
>> drivers/base/power/clock_ops.c, which calls enable_clock():
>>
>>     /**
>>      * enable_clock - Enable a device clock.
>>      * @dev: Device whose clock is to be enabled.
>>      * @con_id: Connection ID of the clock.
>>      */
>>     static void enable_clock(struct device *dev, const char *con_id)
>>     {
>>             struct clk *clk;
>>
>>             clk = clk_get(dev, con_id);
>>             if (!IS_ERR(clk)) {
>>                     clk_prepare_enable(clk);
>>                     clk_put(clk);
>
> This is a violation of the clkdev api as defined in include/linux/clk.h:
>
>         /**
>          * clk_put|------ "free" the clock source
>          * @clk: clock source
>          *
>          * Note: drivers must ensure that all clk_enable calls made on this
>          * clock source are balanced by clk_disable calls prior to calling
>          * this function.

I know.

> So the WARN is doing its job and letting us know about incorrect use of
> the API.
>
>>                     dev_info(dev, "Runtime PM disabled, clock forced on.\n");
>>             }
>>     }
>>
>> I think this affects shmobile, keystone, davinci, omap1, and legacy sh.
>
> Why not keep the reference to the struct clk after get'ing it the first
> time?

And store it where?

dev_pm_get_subsys_data() also depends on CONFIG_PM=y.

Note that there can be multiple clocks.

>> Sorry for not noticing before, we usually build with CONFIG_PM=y.
>> One more reason for making CONFIG_PM=y mandatory on SoCs with clock domains?

On ARM/shmobile, we only use it for the CONFIG_PM=n case, cfr.
drivers/sh/pm_runtime.c. In the CONFIG_PM=y case, we use DT and genpd.

For keystone, davinci, omap1, and legacy sh it's different, though.

With the advent of hardware Power and Clock Domains, keeping support for
CONFIG_PM=n alive is getting harder and harder...

> I don't know about that, but it seems like a reason to fix the clkdev
> usage in the clock domain code.

This is the legacy clock domain code. The way forward is genpd ;-)

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds