Re: [PATCH 2/4] arch/arm/mach-at91/clock.c: Add missing IS_ERR test

From: Ryan Mallon
Date: Mon Jan 24 2011 - 16:51:28 EST

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


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