linux-kernel - Re: [PATCH v4] PM / devfreq: Fix devfreq_add_device() when drivers are built as modules.

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Date:   Wed, 04 Jul 2018 17:26:52 +0900
From:   Chanwoo Choi <cw00.choi@...sung.com>
To:     Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        linux-kernel@...r.kernel.org
Cc:     kernel@...labora.com, Kyungmin Park <kyungmin.park@...sung.com>,
        MyungJoo Ham <myungjoo.ham@...sung.com>,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH v4] PM / devfreq: Fix devfreq_add_device() when drivers
 are built as modules.

Hi Enric,

On 2018년 07월 04일 17:16, Enric Balletbo i Serra wrote:
> Hi Chanwoo,
> 
> On 04/07/18 03:06, Chanwoo Choi wrote:
>> Hi Enric,
>>
>> On 2018년 07월 03일 22:29, Enric Balletbo i Serra wrote:
>>> When the devfreq driver and the governor driver are built as modules,
>>> the call to devfreq_add_device() or governor_store() fails because the
>>> governor driver is not loaded at the time the devfreq driver loads. The
>>> devfreq driver has a build dependency on the governor but also should
>>> have a runtime dependency. We need to make sure that the governor driver
>>> is loaded before the devfreq driver.
>>>
>>> This patch fixes this bug by adding a try_then_request_governor()
>>> function. First tries to find the governor, and then, if it is not found,
>>> it requests the module and tries again.
>>>
>>> Fixes: 1b5c1be2c88e (PM / devfreq: map devfreq drivers to governor using name)
>>> Signed-off-by: Enric Balletbo i Serra <enric.balletbo@...labora.com>
>>> ---
>>>
>>> Changes in v4:
>>> - Kept "locked" devfreq_list from the return of find_devfreq_governor() to
>>>   the unlock of governor_store(). Requested by MyungJoo Ham.
>>>
>>> Changes in v3:
>>> - Remove unneded change in dev_err message.
>>> - Fix err returned value in case to not find the governor.
>>>
>>> Changes in v2:
>>> - Add a new function to request the module and call that function from
>>>   devfreq_add_device and governor_store.
>>>
>>>  drivers/devfreq/devfreq.c | 53 ++++++++++++++++++++++++++++++++++-----
>>>  1 file changed, 47 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
>>> index 0b5b3abe054e..4ea6b19879a1 100644
>>> --- a/drivers/devfreq/devfreq.c
>>> +++ b/drivers/devfreq/devfreq.c
>>> @@ -11,6 +11,7 @@
>>>   */
>>>  
>>>  #include <linux/kernel.h>
>>> +#include <linux/kmod.h>
>>>  #include <linux/sched.h>
>>>  #include <linux/errno.h>
>>>  #include <linux/err.h>
>>> @@ -221,6 +222,46 @@ static struct devfreq_governor *find_devfreq_governor(const char *name)
>>>  	return ERR_PTR(-ENODEV);
>>>  }
>>>  
>>> +/**
>>> + * try_then_request_governor() - Try to find the governor and request the
>>> + *                               module if is not found.
>>> + * @name:	name of the governor
>>
>> Usually, devfreq used 'governor_name' indicating the name of governor.
>> you better to use 'governor_name' instead of 'name' for more readability.
>>
> 
> I don't mind to change if you want. But let me try to convince you first. I used
> name for two reasons:
> 1. I saw that you are using governor_name sometimes, but find_devfreq_governor
> uses name not governor_name. IMHO the function name in these two specific cases
> 'try_then_request_governor(name)' is enough readable.

OK. skip the my comment of changing the variable name. Thanks.

> 2. If we want to use governor_name and then do not have the line exceeding the
> 80 characters I need to split the function in two lines. For me the readability
> is better when you have all in one line.
> 
> If I did not convince you, just let me now and I'll change for governor_name :)
> 

(snip)

-- 
Best Regards,
Chanwoo Choi
Samsung Electronics