linux-kernel - Re: [PATCH v9 5/6] clk: Add floor and ceiling constraints to clock rates

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Message-ID: <54086A81.1060606@collabora.com>
Date:	Thu, 04 Sep 2014 15:34:57 +0200
From:	Tomeu Vizoso <tomeu.vizoso@...labora.com>
To:	Stephen Boyd <sboyd@...eaurora.org>,
	Mike Turquette <mturquette@...aro.org>
CC:	Stephen Warren <swarren@...dotorg.org>,
	Peter De Schrijver <pdeschrijver@...dia.com>,
	linux-kernel@...r.kernel.org, tomasz.figa@...il.com, rabin@....in,
	Thierry Reding <thierry.reding@...il.com>,
	Javier Martinez Canillas <javier.martinez@...labora.co.uk>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v9 5/6] clk: Add floor and ceiling constraints to clock
 rates

On 09/04/2014 01:39 AM, Stephen Boyd wrote:
> On 09/03/14 08:33, Tomeu Vizoso wrote:
>>
>> diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
>> index 61a3492..3a961c6 100644
>> --- a/drivers/clk/clk.c
>> +++ b/drivers/clk/clk.c
>> @@ -560,6 +560,8 @@ struct clk *__clk_create_clk(struct clk_core *clk_core, const char *dev,
>>  	clk->dev_id = dev;
>>  	clk->con_id = con;
>>  
>> +	hlist_add_head(&clk->child_node, &clk_core->per_user_clks);
>> +
> 
> How is this safe with another thread that may be traversing the list? Or
> even two threads calling clk_get_parent() at the same time?

Good point, will take the prepare lock.

>> +int clk_set_floor_rate(struct clk *clk_user, unsigned long rate)
>> +{
>> +	struct clk_core *clk = clk_to_clk_core(clk_user);
>> +
>> +	clk_user->floor_constraint = rate;
>> +	return clk_provider_set_rate(clk, clk_provider_get_rate(clk));
> 
> It would be nice if this was also locked around so that the
> floor_constraint or ceiling_constraint doesn't change while another
> thread is iterating the list. I guess we'll get by though because
> eventually things will settle and either this thread here will set the
> "final" rate, or the other thread in clk_provider_set_rate() will have
> already set the final rate. It just seems wrong to not hold the lock
> while updating what is supposed to be protected by the prepare lock.

Yeah, I also lean towards having an explicit lock, as having a more
deterministic behaviour can be quite helpful when debugging.

Thanks,

Tomeu
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