* Re: [PATCH 2/2] gpio: gpiolib: set gpiochip_remove retval to void
[not found] ` <063D6719AE5E284EB5DD2968C1650D6D17259A1F@AcuExch.aculab.com>
@ 2014-06-10 6:57 ` Andrzej Hajda
0 siblings, 0 replies; only message in thread
From: Andrzej Hajda @ 2014-06-10 6:57 UTC (permalink / raw)
To: David Laight, Lars-Peter Clausen, Ben Dooks
Cc: driverdevel, Linux MIPS Mailing List, spear-devel@list.st.com,
David Daney, Linux-sh list, netdev@vger.kernel.org, Linus Walleij,
patches@opensource.wolfsonmicro.com, linux-wireless,
linux-kernel@vger.kernel.org, platform-driver-x86@vger.kernel.org,
linux-gpio@vger.kernel.org, Geert Uytterhoeven,
linux-arm-kernel@lists.infradead.org, m@bues.ch,
linux-input@vger.kernel.org, linux-samsung-soc
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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