linux-kernel - Re: [RFC][PATCH] bcmai: introduce AI driver

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Date:	Wed, 06 Apr 2011 23:08:32 +0200
From:	Michael Büsch <mb@...sch.de>
To:	Rafał Miłecki <zajec5@...il.com>
Cc:	Arend van Spriel <arend@...adcom.com>,
	"linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
	"John W. Linville" <linville@...driver.com>,
	Larry Finger <Larry.Finger@...inger.net>,
	George Kashperko <george@...u.edu.ua>,
	"b43-dev@...ts.infradead.org" <b43-dev@...ts.infradead.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Russell King <rmk@....linux.org.uk>,
	Arnd Bergmann <arnd@...db.de>,
	linuxdriverproject <devel@...uxdriverproject.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] bcmai: introduce AI driver

On Wed, 2011-04-06 at 23:01 +0200, Rafał Miłecki wrote: 
> W dniu 6 kwietnia 2011 22:57 użytkownik Michael Büsch <mb@...sch.de> napisał:
> > On Wed, 2011-04-06 at 22:42 +0200, Rafał Miłecki wrote:
> >> 2011/4/6 Rafał Miłecki <zajec5@...il.com>:
> >> > If we want to have two drivers working on two (different) cores
> >> > simultaneously, we will have to add trivial mutex to group core
> >> > switching with core operation (read/write).
> >>
> >> With a little of work we could avoid switching and mutexes on no-host
> >> boards. MMIO is not limited to one core at once in such a case.
> >
> > I don't think that this is a problem at all.
> > All that magic does happen inside of the bus I/O handlers.
> > Just like SSB does it.
> > From a driver point of view, the I/O functions just need to
> > be atomic.
> >
> > For SSB it's not always 100% atomic, but we're always safe
> > due to some assumptions being made. But this is an SSB implementation
> > detail that is different from AXI. So don't look too closely
> > at the SSB implementation of the I/O functions. You certainly want
> > to implement them slightly differently in AXI. SSB currently doesn't
> > make use of the additional sliding windows, because they are not
> > available in the majority of SSB devices.
> >
> > The AXI bus subsystem will manage the sliding windows and the driver
> > doesn't know about the details.
> 
> Sure, I've meant mutex inside bcmai (or whatever name), not on the driver side!
> 
> In BCMAI:
> bcmai_read() {
> mutex_get();
> switch_core();
> ioread();
> mutex_release();
> }

Yeah that basically is the idea. But it's a little bit harder than that.
The problem is that the mutex cannot be taken in interrupt context.
A spinlock probably is a bit hairy, too, depending on how heavy
a core switch is on AXI.

On SSB we workaround this with some (dirty but working) assumptions.

On AXI you probably can do lockless I/O, if you use the two windows
(how many windows are there?) in a clever way to avoid core switching
completely after the system was initialized.

-- 
Greetings Michael.

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