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

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Date:	Thu, 07 Apr 2011 00:18:57 +0300
From:	George Kashperko <george@...u.edu.ua>
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


> W dniu 6 kwietnia 2011 23:08 użytkownik Michael Büsch <mb@...sch.de> napisał:
> > 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.
> 
> We have 2 windows. I didn't try this, but let's assume they have no
> limitations. We can use first window for one driver only, second
> driver for second driver only. That gives us 2 drivers simultaneously
> working drivers. No driver need to reset core really often (and not
> inside interrupt context) so we will switch driver's window to agent
> (from core) only at init/reset.
> 
> The question is what amount of driver we will need to support at the same time.
> 

I guess (correct me please, Broadcom guys if I'm wrong) there are two
functions two-head w11 pci host and therefore 4 sliding windows, 2 per
each function.

You really was in need for core switching for PCI SSB hosts, but seem
all that stuff for PCI switching in current bcm80211/utils code is
rudimentary stuff left from PCI times when you was required to use
sliding window for chipcommon and pci bridge core access.

Have nice day,
George



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