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Date:	Mon, 20 Oct 2014 20:44:09 +0300
From:	Vladimir Zapolskiy <>
To:	Alexandre Belloni <>
CC:	Thierry Reding <>,
	Maxime Ripard <>,, Simon <>,,,
Subject: Re: [PATCHv8 1/2] pwm: Add Allwinner SoC support

On 20.10.2014 20:06, Alexandre Belloni wrote:
> On 20/10/2014 at 16:10:31 +0300, Vladimir Zapolskiy wrote :
>> Hi Alexandre,
>> On 20.10.2014 13:29, Alexandre Belloni wrote:
>>> Hi,
>>> On 20/10/2014 at 00:22:57 +0300, Vladimir Zapolskiy wrote :
>>>>> +struct sun4i_pwm_chip {
>>>>> +	struct pwm_chip chip;
>>>>> +	struct clk *clk;
>>>>> +	void __iomem *base;
>>>>> +	struct mutex ctrl_lock;
>>>> why do you use mutex? I haven't found any blocking subcalls under
>>>> protection, a spinlock seems to fit better here.
>>> A mutex here will do the right thing. The lock is never taken in
>>> interrupt context and a mutex is spinning for a few cycles before
>>> putting the thread to sleep.
>> and why do you want to put a thread to sleep in context of the driver?
> Because the PWM is getting configured from either a kernel thread or a
> userspace thread accessing /sys. So you probably want the current thread
> to sleep so the other thread accessing the register can finish. Unless
> you are on smp and then, the mutex will spin for some time and your
> other cpu will be finished by then.

You answer why a thread sleeps, if you use a mutex in the driver. But
you don't answer my question why do you want to put a thread to sleep.

As for me you introduce the excessive heaviness for no reason. There is
no need to sleep while waiting for completion of a register writing.

With best wishes,
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