linux-kernel - Re: [PATCH 01/14] extcon: Add extcon_get_extcon_dev_by_cable

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Date:   Mon, 19 Dec 2016 20:57:06 +0900
From:   Chanwoo Choi <cw00.choi@...sung.com>
To:     Hans de Goede <hdegoede@...hat.com>,
        Sebastian Reichel <sre@...nel.org>,
        Chen-Yu Tsai <wens@...e.org>,
        MyungJoo Ham <myungjoo.ham@...sung.com>
Cc:     "russianneuromancer @ ya . ru" <russianneuromancer@...ru>,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/14] extcon: Add extcon_get_extcon_dev_by_cable_id
 function

Hi,

On 2016년 12월 19일 20:42, Hans de Goede wrote:
> Hi,
> 
> On 19-12-16 11:12, Chanwoo Choi wrote:
>> Hi Hans,
>>
>> On 2016년 12월 19일 09:07, Hans de Goede wrote:
>>> extcon_register_notifier() allows passing in a NULL pointer for the
>>> extcon_device, so that extcon consumers which want extcon events of a
>>> certain type, but do not know the extcon device name (e.g. because
>>> there are different implementation depending on the board), can still
>>> get such events.
>>>
>>> But most drivers will also want to know the initial state of the cable.
>>> Rather then adding NULL edev argument support to extcon_get_state, which
>>> would require looking up the right edev each call, this commit allows
>>> drivers to get the first extcon device with a requested cable-id through
>>> a new extcon_get_extcon_dev_by_cable_id function.
>>>
>>> Signed-off-by: Hans de Goede <hdegoede@...hat.com>
>>> ---
>>>  drivers/extcon/extcon.c | 24 ++++++++++++++++++++++++
>>>  include/linux/extcon.h  |  1 +
>>>  2 files changed, 25 insertions(+)
>>>
>>> diff --git a/drivers/extcon/extcon.c b/drivers/extcon/extcon.c
>>> index 7829846..505c272 100644
>>> --- a/drivers/extcon/extcon.c
>>> +++ b/drivers/extcon/extcon.c
>>> @@ -890,6 +890,30 @@ struct extcon_dev *extcon_get_extcon_dev(const char *extcon_name)
>>>  EXPORT_SYMBOL_GPL(extcon_get_extcon_dev);
>>>
>>>  /**
>>> + * extcon_get_extcon_dev_by_cable_id() - Get an extcon device by a cable id
>>> + * @id:        the unique id of each external connector in extcon enumeration.
>>> + */
>>> +struct extcon_dev *extcon_get_extcon_dev_by_cable_id(unsigned int id)
>>> +{
>>> +    struct extcon_dev *extd;
>>> +    int idx = -EINVAL;
>>> +
>>> +    mutex_lock(&extcon_dev_list_lock);
>>> +    list_for_each_entry(extd, &extcon_dev_list, entry) {
>>> +        idx = find_cable_index_by_id(extd, id);
>>> +        if (idx >= 0)
>>> +            break;
>>> +    }
>>> +    mutex_unlock(&extcon_dev_list_lock);
>>> +
>>> +    if (idx < 0)
>>> +        return NULL;
>>> +
>>> +    return extd;
>>> +}
>>> +EXPORT_SYMBOL_GPL(extcon_get_extcon_dev_by_cable_id);
>>
>> This function do not guarantee the same operation on all of case.
>>
>> For example,
>> The h/w board has multiple extcon provider which support the same external connector. When using this function, this function don't get the correct extcon instance. Just, this function returns the first extcon instance in the registered extcon list.
>>
>> This function has the potential problem.
> 
> True, I wanted this function because I'm afraid that on some
> boards using the axp288_charger.c driver the USB_HOST extcon
> cable may be provided by a different extcon device then the
> "intel-int3496" device, but as you said in your reply to
> 
> "[PATCH 08/14] power: supply: axp288_charger: Actually get and use the USB_HOST extcon device"
> 
> If that happens we can add an array of extcon names to try,
> in axp288_charger.c.
> 
> So I'll modify this patch to use the existing
> extcon_get_extcon_dev with a name argument of "intel-int3496".
> 
> Note that currently extcon_register_notifier accepts a NULL
> edev argument and in that case does pick the first edev with
> a matching cable-id, which has the same problem as you pointed
> out. So perhaps see if anyone actually passes in NULL, and if
> not drop support for a NULL edev argument passed to
> extcon_register_notifier ?

Right. To remove the potential problem,
I'll remove the code in the extcon_register_notfier() function when first parameter is NULL.

-- 
Regards,
Chanwoo Choi