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

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Date:	Thu, 15 Nov 2007 09:43:00 +0100
From:	Haavard Skinnemoen <hskinnemoen@...el.com>
To:	David Brownell <david-b@...bell.net>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	Florian Fainelli <florian.fainelli@...ecomint.eu>
Subject: Re: [patch 2.6.24-rc2 1/3] generic gpio -- gpio_chip support

On Wed, 14 Nov 2007 22:50:17 -0800
David Brownell <david-b@...bell.net> wrote:

> > Since this is the code that runs under the lock  
> 
> No, there's more than that.  This is what runs under it in
> the hot paths, yes, but the gpio request/free paths do
> more work than this.  (That includes direction setting,
> since that can be an implicit request.)

Yeah, I was talking about the hot paths. That's the only place where
raw vs. non-raw performance matters.

> The get/set bit calls are the hot paths.  Locking on those paths
> buys us a consistent locking policy, which is obviously correct.
> It's consistent with the request/free paths.
> 
> But I think what you're suggesting is that the "requested" flag
> is effectively a long-term lock, so grabbing the spinlock on
> those paths is not necessary.  Right?

Exactly. If we add two (quite reasonable) restrictions:
  * The GPIO framework must ensure that GPIO chips cannot be removed
    when one or more pins have been assigned to a client.
  * The client must ensure that it never calls gpio_free()
    simultaneously with gpio_[sg]et_value(), adding locking of its own if
    necessary.

this should be safe.

Håvard
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