linux-kernel - Re: [PATCH v2 1/4] drivers: hwspinlock: add generic framework

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Message-ID: <AANLkTinQdNC0XaOqNyDbdYE6ZcP0QyzpN492j=Kovuea@mail.gmail.com>
Date:	Thu, 25 Nov 2010 08:40:27 +0200
From:	Ohad Ben-Cohen <ohad@...ery.com>
To:	David Brownell <david-b@...bell.net>
Cc:	MugdhaKamoolkar <mugdha@...com>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	Greg KH <greg@...ah.com>, Tony Lindgren <tony@...mide.com>,
	BenoitCousson <b-cousson@...com>,
	Grant Likely <grant.likely@...retlab.ca>,
	HariKanigeri <h-kanigeri2@...com>, SumanAnna <s-anna@...com>,
	Kevin Hilman <khilman@...prootsystems.com>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v2 1/4] drivers: hwspinlock: add generic framework

On Thu, Nov 25, 2010 at 5:59 AM, David Brownell <david-b@...bell.net> wrote:
> My rule of thumb is that nothing is "generic"
> until at least three whatever-it-is instances
> plug in to it.  Sometimes this is called
> the "Rule of Three".
>
> Other than OMAP, what's providing hardware
> spinlocks that plug into this framework?

We are not aware of any.

That's why the first iteration was just an omap-specific misc driver.

But we were asked not to pollute device drivers with omap-specific
interfaces (see the discussion on [1]). I think it's a good goal (it
will keep the IPC drivers that will come from TI platform-agnostic),
so we split the driver into a generic interface plus small
omap-specific implementation.

This way platforms [2] can easily plug into the framework anything
they need to achieve multi-core synchronization. E.g., even in case a
platform doesn't have dedicated silicon, but still need this
functionality, it can still plug in an implementation which is based
on Peterson's shared memory mutual exclusion algorithm (see
http://en.wikipedia.org/wiki/Peterson's_algorithm).

The third alternative is to have this driver completely hidden inside
the omap folders and deliver pdata func pointer to drivers that use
it. I am not fond of this, since the driver really only have a tiny
omap-specific part, and most of it should really sit in drivers/. In
addition, it will probably kill the chance of others using it too.

[1] http://lkml.org/lkml/2010/10/22/317
[2] this is mainly aimed at non-coherent heterogeneous processors that
do not support fancy synchronization primitives

>
> - Dave
>
>
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