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Subject: Re: Regression: bug 85491: radeon 0000:01:00.0: Fatal error during
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From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Bjorn Helgaas <bhelgaas@google.com>
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On Mon, 2014-12-08 at 14:04 -0700, Bjorn Helgaas wrote:
> 
> However, I think 5b28541552ef is taking the wrong approach.  Consider
> a device that, like this Radeon, has a 32-bit prefetchable BAR.  If
> the upstream bridge has a 32-bit prefetchable window, things work as
> expected -- we put the prefetchable BAR in the prefetchable window.
> But if the bridge has a 64-bit prefetchable window, we put that same
> BAR in a 32-bit non-prefetchable window.

Well, that's expected, unless the 64-bit prefetchable window happens to
be fully enclosed in 32-bit space ... So maybe the approach is to check
that this is the case and "downgrade" such 64-bit prefetchable BARs to
32-bit ...

Cheers,
Ben.


> So we upgraded the system with a better bridge, i.e., it supports a
> full 64-bit window instead of just a 32-bit window, and the system
> performs WORSE.  That's what I think is fundamentally broken.
> 
> > Looking at bug#85491 and the logs from Marek, BIOS didn't configure
> windows
> > and BARs for PCI bridge and adapters properly, which forces the
> kernel to
> > reassign those BARs/windows failing to be claimed previously. If I'm
> correct,
> > we need a way to reassign the bridge's non-prefetchable window and
> with it,
> > find_free_bus_resource() needn't check on "!r->parent".
> 
> It's true that the BIOS didn't configure things correctly.  But we
> can't use that as an excuse; the kernel should be able to configure
> things correctly itself because there are plenty of machines where the
> BIOS never configures things.