linux-kernel - Re: [PATCH] revert ath5k ioread32()/iowrite32() usage

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Date:	Wed, 19 Sep 2007 08:18:54 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	"Luis R. Rodriguez" <mcgrof@...il.com>
Cc:	Jiri Slaby <jirislaby@...il.com>, Jeff Garzik <jeff@...zik.org>,
	"John W. Linville" <linville@...driver.com>,
	linux-wireless <linux-wireless@...r.kernel.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] revert ath5k ioread32()/iowrite32() usage - use
	readl()/writel(), we're MMIO-only


> > > > If the driver knows its MMIO, using readX/writeX after pci_iomap() is
> > > > just fine, for all current implementations, and it makes sense that way.
> > >
> > > There is nothing that guarantees this is permitted, any more than there
> > > is anything saying not to use outb/outl. Some of the implementations do
> > > quite strange things. It may happen to work but its not in the
> > > documentation or the comments.
> >
> > I posted a patch to update the documentation with this.
> >
> > > Please, can anybody clarify it?
> >
> > Based on Alan's 'Review-by' remarks on my patch for updating the
> > documenation on pci_iomap() it seems this validates this patch. Please
> > see thread with subject:
> >
> > [PATCH] Clarify pci_iomap() usage for MMIO-only devices
> 
> Well spoke too soon. Now Linus disagrees. We'll see the outcome on the
> other thread then.

To be more precise, a platform has every right to return some kind of
"token" from ioport_map/pci_iomap that encodes the type of address, and
that is -different- from what a normal ioremap does. In which case, you
will -not- be able to use readb/writeb & cie on such a token.

The fact that current implementations seem to return something for MMIO
that is equivalent to what ioremap returns is an accident and cannot be
relied upon.

Cheers,
Ben.


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