linux-kernel - Re: [PATCH] acpi-cpufreq: Add a miss ifdef CONFIG_X86_ACPI_CPUFREQ

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Date:	Tue, 14 Jul 2015 10:09:47 +0800
From:	Pan Xinhui <xinhuix.pan@...el.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
CC:	"Rafael J. Wysocki" <rafael@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	"yanmin_zhang@...ux.intel.com" <yanmin_zhang@...ux.intel.com>,
	"mnipxh@....com" <mnipxh@....com>
Subject: Re: [PATCH] acpi-cpufreq: Add a miss ifdef CONFIG_X86_ACPI_CPUFREQ_CPB

hi, Rafael,
	thanks for you reply :)
On 2015年07月14日 07:26, Rafael J. Wysocki wrote:
> On Monday, July 13, 2015 02:33:08 PM Pan Xinhui wrote:
>> hi, Rafeal
>> 	thanks for your reply. :)
>>
>> On 2015年07月11日 04:44, Rafael J. Wysocki wrote:
>>> Hi,
>>>
>>> On Fri, Jul 10, 2015 at 7:50 AM, Pan Xinhui <xinhuix.pan@...el.com> wrote:
>>>>
>>>> If CONFIG_X86_ACPI_CPUFREQ_CPB has not been defined, the placeholder for
>>>> cpb is not needed. Add ifdef around it.
>>>>
>>>> Signed-off-by: Pan Xinhui <xinhuix.pan@...el.com>
>>>> ---
>>>>  drivers/cpufreq/acpi-cpufreq.c | 2 ++
>>>>  1 file changed, 2 insertions(+)
>>>>
>>>> diff --git a/drivers/cpufreq/acpi-cpufreq.c b/drivers/cpufreq/acpi-cpufreq.c
>>>> index e7fcaa6..314a19e 100644
>>>> --- a/drivers/cpufreq/acpi-cpufreq.c
>>>> +++ b/drivers/cpufreq/acpi-cpufreq.c
>>>> @@ -884,7 +884,9 @@ static int acpi_cpufreq_resume(struct cpufreq_policy *policy)
>>>>  static struct freq_attr *acpi_cpufreq_attr[] = {
>>>>         &cpufreq_freq_attr_scaling_available_freqs,
>>>>         &freqdomain_cpus,
>>>> +#ifdef CONFIG_X86_ACPI_CPUFREQ_CPB
>>>>         NULL,   /* this is a placeholder for cpb, do not remove */
>>>> +#endif
>>>
>>> Adding the ifdef here doesn't change anything, because the next NULL
>>> will play the role of the one you've just #ifdefed and the structure
>>> will be filled with zeros from that point on anyway.
>>>
>> Yes, adding ifdef here does not change any binary codes. But I want to make the codes more readable. :)
>> Patch author has noticed two *NULL* here would confuse people, especially who first read this acpi-cpufreq.c file
>> From code style point, it would be better to have #ifdef around it. 
> 
> Not really.
> 
> Why don't you simply drop *both* NULLs?
> 
Just like string end with *NULL* :)

1021 static int cpufreq_add_dev_interface(struct cpufreq_policy *policy,
1022                      struct device *dev)
1023 {
1024     struct freq_attr **drv_attr;
1025     int ret = 0;
1026 
1027     /* set up files for this cpu device */
1028     drv_attr = cpufreq_driver->attr;
1029     while (drv_attr && *drv_attr) {
1030         ret = sysfs_create_file(&policy->kobj, &((*drv_attr)->attr));
1031         if (ret)
1032             return ret;
1033         drv_attr++;
1034     }
If struct freq_attr *acpi_cpufreq_attr[] did not end with NULL, line 1033 will access invalid data area.
If *drv_attr(the data after struct freq_attr * array[]) happened to be not NULL. panic may hit in sysfs_create_file :(
So at least one *NULL* must be in the end of freq_attr *array[].

Actually in acpi-cpufreq.c, in acpi_cpufreq_init function.
 957         struct freq_attr **iter;
 958 
 959         pr_debug("adding sysfs entry for cpb\n");
 960 
 961         for (iter = acpi_cpufreq_attr; *iter != NULL; iter++)
 962             ;
 963 
 964         /* make sure there is a terminator behind it */
 965         if (iter[1] == NULL)
 966             *iter = &cpb;
 967     }
line965, check of iter[1] is not needed. Maybe the patch author was afraid of an unexpected remove of first *NULL*.
It might be a better solution to add ifdef CONFIG_X86_ACPI_CPUFREQ_CPB around that *NULL*, and remove this !iter[1] check.

thanks
xinhui

> 
>>
>>> You'd need to #ifdef it in the struct freq_attr definition, but I'm
>>> not sure it's worth the effort.
>>>
>>
>> struct freq_attr *cpb* is defined in #ifdef section. :)
> 
> Ah, OK.
> 
> 
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