linux-kernel - Re: [RFC,PATCH 1/2] Add a common struct clk

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Message-ID: <20100603110533.GB7127@n2100.arm.linux.org.uk>
Date:	Thu, 3 Jun 2010 12:05:33 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Jeremy Kerr <jeremy.kerr@...onical.com>
Cc:	Ben Dooks <ben-linux@...ff.org>,
	Ben Herrenchmidt <benh@...nel.crashing.org>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC,PATCH 1/2] Add a common struct clk

On Thu, Jun 03, 2010 at 06:24:50PM +0800, Jeremy Kerr wrote:
> OK, this would mean adding parent to struct clk:
> 
> struct clk {
> 	struct clk_operations	*ops;
> 	atomic_t				enable_count;

I don't think it makes sense for this to be an atomic type.

> static inline int clk_enable(struct clk *clk)
> {
> 	int ret;
> 
> 	if (atomic_inc(clk->enable_count) != 1))
> 		return 0;

Okay, so what if we sleep at this point, and someone else comes along
and calls clk_enable(), which'll merely increment the count and return.

Unfortunately, the clock is still disabled at that point - which is a
violation of what's supposed to happen.

Or to put it another way, the above method results in clk_enable()
sometimes returning with the clock enabled and sometimes with the
clock still disabled.

That's not nice behaviour for drivers which may need the clock enabled
to read/write the device registers.
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