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

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Date:	Wed, 21 Oct 2015 18:46:35 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Michael Turquette <mturquette@...libre.com>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	"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 Wed, Oct 21, 2015 at 5:50 PM, Michael Turquette
<mturquette@...libre.com> wrote:
> Quoting Russell King - ARM Linux (2015-10-21 03:59:32)
>> On Wed, Oct 21, 2015 at 11:50:07AM +0200, Geert Uytterhoeven wrote:
>> > On Tue, Oct 20, 2015 at 2:40 PM, Michael Turquette
>> > <mturquette@...libre.com> wrote:
>> > > Why not keep the reference to the struct clk after get'ing it the first
>> > > time?
>> >
>> > And store it where?
>>
>> Not my problem :)
>>
>> Users are supposed to hold on to the reference obtained via clk_get()
>> while they're making use of the clock: in some implementations, this
>> increments the module use count if the clock driver is a module, and
>> may have other effects too.
>>
>> Dropping that while you're still requiring the clock to be enabled is
>> unsafe: if it is provided by a module, then removing and reinserting
>> the module may very well change the enabled state of the clock, it
>> most certainly will disrupt the enable count.
>>
>> It's always been this way, right from the outset, and when I've seen
>> people doing this bollocks, I've always pointed out that it's wrong.
>> Generally, people will fix it once they become aware of it, so it's
>> really that people just don't like reading and conforming to published
>> API requirements.
>>
>> I think the root cause is that people just don't like reading what
>> other people write in terms of documentation, and they prefer to go
>> off and do their own thing, provided it works for them.
>
> Right, so in other words this problem must be solved by the caller of
> clk_get, as it should be. I have never much looked at the pm clk code in
> question, but I gave it a quick look and came up with some example code
> that does not compile, in an effort to be as helpful as possible.
>
> Basically I added a flex array to struct pm_clk_notifier_block, so that
> now there are two flex arrays which is stupid. But I am too lazy to
> replace the nbclk->clks thing with a malloc after walking all of the
> clk_id's to figure out the number of clocks. Or we could just add
> .num_clk to the struct, fix up all 4 users of it and drop the NULL
> sentinel used the .clk_id's... Hmm that would have been better.

Thanks for trying.

> index 25266c6..45e58fe 100644
> --- a/include/linux/pm_clock.h
> +++ b/include/linux/pm_clock.h
> @@ -16,6 +16,7 @@ struct pm_clk_notifier_block {
>         struct notifier_block nb;
>         struct dev_pm_domain *pm_domain;
>         char *con_ids[];
> +       struct clk *clks[];
>  };
>
>  struct clk;

Unfortunately that won't work: while the .con_ids[] array is per-platform,
the .clks[] array should be per-device. I.e. it should be tied to the struct
device, not to the struct pm_clk_notifier_block.

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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