Date:	Tue, 25 Jan 2011 10:51:38 +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 10:31 AM, Julia Lawall wrote:
> On Tue, 25 Jan 2011, Ryan Mallon wrote:
> 
>> On 01/25/2011 10:01 AM, Julia Lawall wrote:
>>> On Tue, 25 Jan 2011, Ryan Mallon wrote:
>>>
>>>> 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.
>>>
>>> Right, that was what I was worried about.  That one would find the same 
>>> information already present but somewhere else.  But perhaps it can't 
>>> happen, or it doesn't matter if it does?
>>
>> I think that users are expected to ensure that clock names and clock
>> function names do not overlap.
> 
> One can't have a clock with a different name but the same device and 
> function?

You could, but it would not be helpful. Clock associations are used so
that _different_ devices can have the same function and map to the
correct clock. This is used when there are multiple instances of a
single peripheral. For example, the uart clocks work like this:

	at91_clock_associate("usart1_clk", &pdev->dev, "usart");

so then you can do this in a driver:
	
	uart_clk = clk_get(&pdev->dev, "usart");

Rather than:

	uart_clk = clk_get(NULL, "usart1_clk");

The former will find the correct uart clock for the device. Because each
uart is a separate device the correct clock will be selected for each uart.
	
My point was that there should be no overlap between clk->name and
clk->function otherwise clk_get will not be able to return the correct
clock.

~Ryan

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