linux-kernel - Re: [patch 2.6.24-rc2 1/3] generic gpio -- gpio

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Date:	Thu, 15 Nov 2007 08:32:43 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	David Brownell <david-b@...bell.net>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	Florian Fainelli <florian.fainelli@...ecomint.eu>,
	Haavard Skinnemoen <hskinnemoen@...el.com>,
	Ingo Molnar <mingo@...e.hu>,
	Nick Piggin <nickpiggin@...oo.com.au>
Subject: Re: [patch 2.6.24-rc2 1/3] generic gpio -- gpio_chip support

On Wed, 14 Nov 2007, David Brownell wrote:
> > The protection of the chip list can be converted to a mutex and
> > does not need to be a spinlock at all.
> 
> No, we still need to use a spinlock to protect table changes.
> The reason for that is briefly:
> 
>   - gpio_request()/gpio_free() have so far been optional.  Most
>     platforms implement them as NOPs, not all drivers use them.
>     (Having gpiolib in place should help change that ...)

By magically doing the request of the pin ? See below.

>   - gpio_direction_input()/gpio_direction_output() implicitly
>     request the pins, if they weren't already requested.

Eek, that's completely wrong. Allowing to access a resource _before_
it is assigned and then doing the assignment implicit is a really bad
idea.

>   - Those input/output direction-setting calls may be called
>     in IRQ contexts, which means (on non-RT kernels) no mutex.

There is no reason to do that if you actually have a useful reference
to the chip _before_ accessing the pin.
 
> So we're actually in good shape; just take out a bit of code
> (or turn it into debugging instrumentation) and I don't think
> anyone will complain about the locking any more.

This still does not solve the lookup, which is done for each operation
on a pin (direction setting, read, write).

     	 tglx

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