linux-kernel - Re: [PATCH v2 3/3] power: supply: bq24735-charger: allow chargers to share the ac-detect gpio

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Date:   Mon, 9 Jan 2017 19:55:48 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Peter Rosin <peda@...ntia.se>
Cc:     Sebastian Reichel <sre@...nel.org>,
        Alexandre Courbot <gnurou@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>
Subject: Re: [PATCH v2 3/3] power: supply: bq24735-charger: allow chargers to
 share the ac-detect gpio

On Mon, Jan 2, 2017 at 9:31 AM, Peter Rosin <peda@...ntia.se> wrote:
> On 2016-12-30 08:49, Linus Walleij wrote:
>> On Wed, Dec 14, 2016 at 6:41 PM, Peter Rosin <peda@...ntia.se> wrote:
>>> On 2016-12-14 18:01, Sebastian Reichel wrote:
>>>> [of course I forgot to actually add gpio people, let's try again]
>>>>
>>>> On Wed, Dec 14, 2016 at 05:59:21PM +0100, Sebastian Reichel wrote:
>>>>> Hi,
>>>>>
>>>>> On Wed, Dec 14, 2016 at 12:56:45AM +0100, Peter Rosin wrote:
>>>>>> If several parallel bq24735 chargers have their ac-detect gpios wired
>>>>>> together (or if only one of the parallel bq24735 chargers have its
>>>>>> ac-detect pin wired to a gpio, and the others are assumed to react the
>>>>>> same), then all driver instances need to check the same gpio. But the
>>>>>> gpio subsystem does not allow sharing gpios, so handle that locally.
>>>>>
>>>>> Adding GPIO subsystem people to see if they can come up with
>>>>> something in the gpiod API for this usecase.
>>>
>>> Right, I don't like how my new code steps away from gpio descriptors.
>>
>> The issue of shared gpiods have come up over and over again.
>> For example the messy regulator code needs this too.
>>
>> It is better if we implement something like gpiod_get_shared()
>> in the gpiolib of these cases.
>>
>> Just put a refcount in struct gpio_desc in drivers/gpio/gpiolib.h
>> for this case I guess?
>
> I actually tried that, but ran into atomicy issues with the
> FLAG_REQUESTED bit and gave up. Didn't really try all that hard
> though, but I simply didn't feel comfortable with going near such
> fundamental designs...

Oh I see. Well if it is of any help that would make me nervous too.

I would just remove the use of FLAG_REQUESTED altogether,
redefine the flags in gpiolib.h from 0 and add a struct kref into the struct
to deal with the refcounting.

That should do it. I think.

Yours,
Linus Walleij