linux-kernel - Re: [PATCH] clk: <linux/clk.h>: correct clk_get

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Date:   Tue, 27 Apr 2021 11:10:42 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     linux-kernel@...r.kernel.org, linux-clk@...r.kernel.org
Subject: Re: [PATCH] clk: <linux/clk.h>: correct clk_get_parent()
 documentation

On 4/27/21 11:03 AM, Russell King - ARM Linux admin wrote:
> On Tue, Apr 27, 2021 at 10:07:04AM -0700, Randy Dunlap wrote:
>> On 4/27/21 2:38 AM, Russell King - ARM Linux admin wrote:
>>> On Mon, Apr 26, 2021 at 08:13:42PM -0700, Randy Dunlap wrote:
>>>> Make the kernel-doc return value agree with both the stub implementation
>>>> in <linux/clk.h> and the non-stub function in drivers/clk/clk.c.
>>>>
>>>> Signed-off-by: Randy Dunlap <rdunlap@...radead.org>
>>>> Cc: Russell King <linux@...linux.org.uk>
>>>> Cc: linux-clk@...r.kernel.org
>>>> ---
>>>> There are several drivers that call clk_get_parent() and use its
>>>> return value as though it returns an ERR_PTR(). I am working on a
>>>> patch series to fix those.
>>>
>>> The whole premise of the CLK API is that:
>>>
>>> - clk values that IS_ERR() returns true are errors
>>> - clk values that IS_ERR() returns false are valid
>>>
>>> and that is that - nice, simple, easy to understand. So the
>>> documentation is correct. If clk_get_parent() needs to return an
>>> error, it does so by returning an error-pointer, not by returning
>>> NULL.
>>
>> Hm, so the 6 drivers that do this are OK then, if
>> clk_get_parent() returns NULL?
>>
>>
>> 	parent = clk_get_parent(clk);
>> 	if (IS_ERR(parent)) {
>> 		dev_err(cpu_dev, "Cannot get parent clock for CPU0\n");
>> 		clk_put(clk);
>> 		return PTR_ERR(parent);
>> 	}
>>
>> 	/* Get parent CPU frequency */
>> 	base_frequency =  clk_get_rate(parent);
>>
>> boom?
> 
> As I say, the premise is that only clocks where IS_ERR(clk) is true
> are an error. Everything else must be treated as an acceptable clock.
> 
> Sometimes the NULL clock is used for "I don't have a clock", and that
> must not cause the API to go "boom".
> 
> In the case of the CCF implementation, clk_get_rate(NULL) will return
> 0.
> 

Thank you for the clarification. That is helpful.

-- 
~Randy