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Re: [PATCH 2/2] gpio: gpiolib: set gpiochip_remove retval to void

To: David Laight <David.Laight@ACULAB.COM>, Lars-Peter Clausen <lars@metafoo.de>, Ben Dooks <ben@trinity.fluff.org>
Subject: Re: [PATCH 2/2] gpio: gpiolib: set gpiochip_remove retval to void
From: Andrzej Hajda <a.hajda@samsung.com>
Date: Tue, 10 Jun 2014 08:57:32 +0200
Cc: driverdevel <devel@driverdev.osuosl.org>, Linux MIPS Mailing List <linux-mips@linux-mips.org>, "spear-devel@list.st.com" <spear-devel@list.st.com>, David Daney <ddaney.cavm@gmail.com>, Linux-sh list <linux-sh@vger.kernel.org>, "netdev@vger.kernel.org" <netdev@vger.kernel.org>, Linus Walleij <linus.walleij@linaro.org>, "patches@opensource.wolfsonmicro.com" <patches@opensource.wolfsonmicro.com>, linux-wireless <linux-wireless@vger.kernel.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "platform-driver-x86@vger.kernel.org" <platform-driver-x86@vger.kernel.org>, "linux-gpio@vger.kernel.org" <linux-gpio@vger.kernel.org>, Geert Uytterhoeven <geert@linux-m68k.org>, "linux-arm-kernel@lists.infradead.org" <linux-arm-kernel@lists.infradead.org>, "m@bues.ch" <m@bues.ch>, "linux-input@vger.kernel.org" <linux-input@vger.kernel.org>, linux-samsung-soc@vger.kernel.org
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On 06/09/2014 03:43 PM, David Laight wrote:
> From: Of Andrzej Hajda
> ...
>>> You can't error out on module unload, although that's not really relevant
>>> here. gpiochip_remove() is typically called when the device that registered
>>> the GPIO chip is unbound. And despite some remove() callbacks having a
>>> return type of int you can not abort the removal of a device.
>>
>> It is a design flaw in many subsystems having providers and consumers,
>> not only GPIO. The same situation is with clock providers, regulators,
>> phys, drm_panels, ..., at least it was such last time I have tested it.
>>
>> The problem is that many frameworks assumes that lifetime of provider is
>> always bigger than lifetime of its consumers, and this is wrong
>> assumption - usually it is not possible to prevent unbinding driver from
>> device, so if the device is a provider there is no way to inform
>> consumers about his removal.
>>
>> Some solution for such problems is to use some kind of availability
>> callbacks for requesting resources (gpios, clocks, regulators,...)
>> instead of simple 'getters' (clk_get, gpiod_get). Callbacks should
>> guarantee that the resource is always valid between callback reporting
>> its availability and callback reporting its removal. Such approach seems
>> to be complicated at the first sight but it should allow to make the
>> code safe and as a bonus it will allow to avoid deferred probing.
>> Btw I have send already RFC for such framework [1].
> 
> Callbacks for delete are generally a locking nightmare.
> A two-way handshake is also usually needed to avoid problems
> with concurrent disconnect requests.

The framework I have proposed is lock-less[1] and concurrent requests
are serialized so both objections are invalid.

[1]: in fact the framework uses spinlock, but only to protect internal
list simple operations, and even this could be converted to fully
lock-less implementation.

Andrzej

> 
>       David
> 


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