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

To: 'Andrzej Hajda' <a.hajda@samsung.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: David Laight <David.Laight@ACULAB.COM>
Date: Mon, 9 Jun 2014 13:43:56 +0000
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Cc: Linux MIPS Mailing List <linux-mips@linux-mips.org>, "m@bues.ch" <m@bues.ch>, Linux-sh list <linux-sh@vger.kernel.org>, Linus Walleij <linus.walleij@linaro.org>, "platform-driver-x86@vger.kernel.org" <platform-driver-x86@vger.kernel.org>, "linux-leds@vger.kernel.org" <linux-leds@vger.kernel.org>, driverdevel <devel@driverdev.osuosl.org>, Alexandre Courbot <gnurou@gmail.com>, "patches@opensource.wolfsonmicro.com" <patches@opensource.wolfsonmicro.com>, "linux-samsungsoc@vger.kernel.org" <linux-samsungsoc@vger.kernel.org>, Geert Uytterhoeven <geert@linux-m68k.org>, "linux-input@vger.kernel.org" <linux-input@vger.kernel.org>, Linux Media Mailing List <linux-media@vger.kernel.org>, "spear-devel@list.st.com" <spear-devel@list.st.com>, "linux-gpio@vger.kernel.org" <linux-gpio@vger.kernel.org>, David Daney <ddaney.cavm@gmail.com>, "linux-arm-kernel@lists.infradead.org" <linux-arm-kernel@lists.infradead.org>, "netdev@vger.kernel.org" <netdev@vger.kernel.org>, linux-wireless <linux-wireless@vger.kernel.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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Thread-topic: [PATCH 2/2] gpio: gpiolib: set gpiochip_remove retval to void
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.

        David




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