linux-kernel - Re: [PATCH v13 4/6] clk: Add rate constraints to clocks

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Date:	Thu, 29 Jan 2015 11:13:38 -0800
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Tomeu Vizoso <tomeu.vizoso@...labora.com>,
	Mike Turquette <mturquette@...aro.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Javier Martinez Canillas <javier.martinez@...labora.co.uk>,
	Jonathan Corbet <corbet@....net>,
	Tony Lindgren <tony@...mide.com>,
	Russell King <linux@....linux.org.uk>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Boris Brezillon <boris.brezillon@...e-electrons.com>,
	Emilio López <emilio@...pez.com.ar>,
	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Tero Kristo <t-kristo@...com>,
	Manuel Lauss <manuel.lauss@...il.com>,
	Alex Elder <elder@...aro.org>,
	Matt Porter <mporter@...aro.org>,
	Haojian Zhuang <haojian.zhuang@...aro.org>,
	Zhangfei Gao <zhangfei.gao@...aro.org>,
	Bintian Wang <bintian.wang@...wei.com>,
	Chao Xie <chao.xie@...vell.com>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Linux MIPS Mailing List <linux-mips@...ux-mips.org>,
	Linux-sh list <linux-sh@...r.kernel.org>
Subject: Re: [PATCH v13 4/6] clk: Add rate constraints to clocks

On 01/29/15 05:31, Geert Uytterhoeven wrote:
> Hi Tomeu, Mike,
>
> On Fri, Jan 23, 2015 at 12:03 PM, Tomeu Vizoso
> <tomeu.vizoso@...labora.com> wrote:
>> --- a/drivers/clk/clk.c
>> +++ b/drivers/clk/clk.c
>> @@ -2391,25 +2543,24 @@ int __clk_get(struct clk *clk)
>>         return 1;
>>  }
>>
>> -static void clk_core_put(struct clk_core *core)
>> +void __clk_put(struct clk *clk)
>>  {
>>         struct module *owner;
>>
>> -       owner = core->owner;
>> +       if (!clk || WARN_ON_ONCE(IS_ERR(clk)))
>> +               return;
>>
>>         clk_prepare_lock();
>> -       kref_put(&core->ref, __clk_release);
>> +
>> +       hlist_del(&clk->child_node);
>> +       clk_core_set_rate_nolock(clk->core, clk->core->req_rate);
> At this point, clk->core->req_rate is still zero, causing
> cpg_div6_clock_round_rate() to be called with a zero "rate" parameter,
> e.g. on r8a7791:

Hmm.. I wonder if we should assign core->req_rate to be the same as
core->rate during __clk_init()? That would make this call to
clk_core_set_rate_nolock() a nop in this case.

>
> cpg_div6_clock_round_rate: clock sd2 rate 0 parent_rate 780000000 div 1
> cpg_div6_clock_round_rate: clock sd1 rate 0 parent_rate 780000000 div 1
> cpg_div6_clock_round_rate: clock mmc0 rate 0 parent_rate 780000000 div 1
> cpg_div6_clock_round_rate: clock sd1 rate 0 parent_rate 780000000 div 1
> cpg_div6_clock_round_rate: clock sd1 rate 0 parent_rate 780000000 div 1
> cpg_div6_clock_round_rate: clock sd2 rate 0 parent_rate 780000000 div 1
> cpg_div6_clock_round_rate: clock sd2 rate 0 parent_rate 780000000 div 1
>
> and cpg_div6_clock_calc_div() is called to calculate
>
>         div = DIV_ROUND_CLOSEST(parent_rate, rate);
>
> Why was this call to clk_core_set_rate_nolock() in __clk_put() added?
> Before, there was no rate setting done at this point, and
> cpg_div6_clock_round_rate() was not called.

We need to call clk_core_set_rate_nolock() here to drop any min/max rate
request that this consumer has.

>
> Have the semantics changed? Should .round_rate() be ready to
> accept a "zero" rate, and use e.g. the current rate instead?

It seems like we've also exposed a bug in cpg_div6_clock_calc_div().
Technically any driver could have called clk_round_rate() with a zero
rate before this change and that would have caused the same division by
zero.

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project

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