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SubjectRe: [Gta04-owner] [PATCH 5/6] phy: twl4030-usb: add support for reading resistor on ID pin.
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Hi,

Am 02.06.2015 um 22:11 schrieb Pavel Machek <pavel@ucw.cz>:

> On Tue 2015-06-02 16:06:47, Dr. H. Nikolaus Schaller wrote:
>> Hi,
>>
>> Am 02.06.2015 um 15:49 schrieb Kishon Vijay Abraham I <kishon@ti.com>:
>>
>>> Hi,
>>>
>>> On Tuesday 02 June 2015 03:07 AM, NeilBrown wrote:
>>>> On Mon, 1 Jun 2015 19:06:52 +0530 Kishon Vijay Abraham I <kishon@ti.com>
>>>> wrote:
>>>>
>>>>> Hi,
>>>>>
>>>>> On Thursday 16 April 2015 01:33 PM, NeilBrown wrote:
>>>>>> From: NeilBrown <neilb@suse.de>
>>>>>>
>>>>>> The twl4030 phy can measure, with low precision, the
>>>>>> resistance-to-ground of the ID pin.
>>>>>>
>>>>>> Add a function to read the value, and export the result
>>>>>> via sysfs.
>>>>>
>>>>> Little sceptical about adding new sysfs entries. Do you have a good reason to
>>>>> add this?
>>>>
>>>> The hardware can report the value, so why not present it to user-space?
>>>>
>>>> I originally used this with a udev rule which would configure the maximum
>>>> current based on the resistance measure - to work with the particular charger
>>>> hardware I have.
>>>>
>>>> More recent patches try to do all of the max-current configuration in the
>>>> kernel, so I could live without exporting the value via sysfs if that is a
>>>> show-stopper.
>>>>
>>>> I can't see where the scepticism comes from though. It is a well defined
>>>> and cleary documented feature of the hardware. Why not expose it?
>
> Is it well defined enough that it will work on other chargers, too?

It reports the resistance of the charger’s ID pin. So that works for all chargers connected
to a twl4030. As long as the ID pin goes to a 5 pin USB socket.

Other charger drivers do not need to expose a similar attribute since each twl4030 has its
unique path within the /sys tree.

>
>>> ABI can never be removed or modified later. So should be really careful before adding it.
>>
>> Is /sys considered ABI?
>
> Yes.

You are right: https://lwn.net/Articles/172986/

But I am as well with my doubts: https://lwn.net/Articles/173093/

>
>> User space developers are always reminded not to rely on /sys nodes.
>
> No.

Then please explain why I have the impression that it is quite unstable.

BR,
Nikolaus



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