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

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Date:	Wed, 30 Sep 2015 17:38:46 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Michael Turquette <mturquette@...libre.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-clk <linux-clk@...r.kernel.org>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Lee Jones <lee.jones@...aro.org>,
	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Sekhar Nori <nsekhar@...com>,
	Kevin Hilman <khilman@...nel.org>,
	Santosh Shilimkar <ssantosh@...nel.org>,
	Tony Lindgren <tony@...mide.com>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Linux-sh list <linux-sh@...r.kernel.org>,
	Linux PM list <linux-pm@...r.kernel.org>
Subject: Re: [PATCH RFC RFT 2/3] clk: clk_put WARNs if user has not disabled clk

Hi Mike,

On Fri, Aug 7, 2015 at 9:09 PM, Michael Turquette
<mturquette@...libre.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@...libre.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);
                    dev_info(dev, "Runtime PM disabled, clock forced on.\n");
            }
    }

I think this affects shmobile, keystone, davinci, omap1, and legacy sh.

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?

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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
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