linux-kernel - Re: [PATCH 2/4] arch/arm/mach-at91/clock.c: Add missing IS

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Date:	Tue, 25 Jan 2011 09:38:51 +1300
From:	Ryan Mallon <ryan@...ewatersys.com>
To:	Julia Lawall <julia@...u.dk>
CC:	Vasiliy Kulikov <segooon@...il.com>,
	Russell King <linux@....linux.org.uk>,
	kernel-janitors@...r.kernel.org,
	Nicolas Ferre <nicolas.ferre@...el.com>,
	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>,
	Andrew Victor <linux@...im.org.za>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] arch/arm/mach-at91/clock.c: Add missing IS_ERR test

On 01/25/2011 09:28 AM, Julia Lawall wrote:
>> Julia is correct. Some architectures can return NULL from clk_get, but I
>> didn't check the at91 before posting :-/. If we can't return NULL from
>> clk_get then we shouldn't bother checking for it. I do think we should
>> drop the !IS_ERR(clk_get(dev, func)) check though.
> 
> It seems a bit subtle, because the clk manipulated by clk_get in the call 
> of clk_get(dev, func) is not necessarily the same as the one in 
> clock_associate.  But perhaps this is the only possibility in practice?

Not sure I follow. The at91 clk_get does not modify the clk. In
at91_clock_associate we have:

	clk->function = func;
	clk->dev = dev;

and in clk_get we have:

	if (clk->function && (dev == clk->dev) &&
		strcmp(id, clk->function) == 0)
            return clk;

So at91_clock_associate sets the function for a clock, and clk_get
returns clocks based on the function association if the name lookup
fails. The only caveat to this is that the the clock function name
(clk->function) is not the same as any others clock's clk->name.

The !IS_ERR(clk_get(dev, func)) check in at91_clock_associate just
appears to check that the clock is not already associated with the given
function. We don't really need this check because we are just making the
same assignment anyway.

~Ryan

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