Access to PCI Expansion ROMs on PPC

Benjamin Herrenschmidt benh at
Mon Nov 26 07:20:00 EST 2007

On Sun, 2007-11-25 at 08:30 -0500, Jon Smirl wrote:
> > The two cards with x86 firmware return 0xFF for those two readb()
> > instructions, while the X1900 with OF returns 0x00 for the readb().
> >
> > Could one of the more knowledgeable folk for PPC intricacies suggest
> why
> > those readb calls are returning the wrong data?
> I don't know PPC at this low of level but it may be a problem with non
> word-aligned access to memory. I thought readb() was supposed to work
> on all archs and alignment issues are handled inside readb(). Also,
> the readw() may have an endian bug.

Ugh ? Read be reads -bytes- ! Can you tell me how the hell can a byte
access be non-aligned ?

> BenH, has source code for an x86 emulator that will run on PPC. That
> will let you run the ROMs. The original plan was for the kernel to
> generate a uevent that would have triggered the x86 emulator to run.
> Progress along that path was blocked by the X developers. The X server
> contains code for enabling the PCI ROM and reading it from user space.
> I wanted to move this code out of X and into the kernel.
> Because the path was blocked things like the PCI ROM API were never
> throughly tested. It works most of the time but the occasional problem
> is still turning up. Once we identify the PPC problem we can fix it in
> the kernel.

Nobody blocked anything, you are free to develop an emulator triggered
by a uevent, nobody prevented you from doing so.

Now, regarding that user problem, this it totally unrelated as it's a
"mac" card.

It's possible that it's one of these radeons that disable ROM access via
a register in which case a quirk is needed to re-enable it.


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