linux-kernel - Re: [PATCH 1/3] fbmem: fix aperture overlapping check

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Date:	Tue, 13 Apr 2010 06:28:21 +1000
From:	Dave Airlie <airlied@...hat.com>
To:	Marcin Slusarz <marcin.slusarz@...il.com>
Cc:	linux-fbdev@...r.kernel.org, nouveau@...ts.freedesktop.org,
	LKML <linux-kernel@...r.kernel.org>,
	dri-devel <dri-devel@...ts.freedesktop.org>,
	Peter Jones <pjones@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/3] fbmem: fix aperture overlapping check

On Mon, 2010-04-12 at 13:34 +0200, Marcin Slusarz wrote:
> On Mon, Apr 12, 2010 at 09:54:28AM +1000, Dave Airlie wrote:
> > On Sat, 2010-04-10 at 21:55 +0200, marcin.slusarz@...il.com wrote:
> > > fb_do_apertures_overlap is returning wrong value when one aperture
> > > is completely whithin the other. Add generic ranges_overlap macro
> > > (probably kernel.h candidate) and use it here.
> > > 
> > 
> > That doesn't seem right.
> > 
> > The rules are:
> > 
> > the generic aperture has to be equal or smaller than the hw aperture,
> > otherwise the generic driver will be trashing random hw pieces on the
> > machine.

> Why "it has to"? Why generic aperture can't start one byte before hw?

Think about how it works a bit more, the answer is obvious.

The hw aperture is defined by the actual hardware, not by the OS or my
code. The PCI apertures are well defined. Now for a generic driver like
vesa or offb to access the hardware it needs to access the hw apertures.

It can't go accessing one byte before the hw aperture, as that will
crash the machine, or write to some other devices hw.

> 
> > So with that in mind, the check makes sure the generic aperture starts
> > somewhere inside the hw aperture, which the test clearly gets right.
> 
> So your only objection is that it's impossible with the current code?
> 
> > Have you got a pointer to a machine where it fails?
> 
> No, it failed with an artifical test while I was working on vga16fb handoff
> (unfinished).

You won't be able to make this work for vga16fb from what I can see
since it access 0xa000 directly, not via any of the defined apertures
that vesafb/offb use. vga16fb will need a different approach I suspect.

Dave.


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