linux-kernel - Re: [PATCH v2] PCI: Always set prefetchable base/limit upper32 registers

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Date:	Mon, 30 Nov 2009 21:10:54 -0700
From:	Alex Williamson <alex.williamson@...com>
To:	Grant Grundler <grundler@...isc-linux.org>
Cc:	jbarnes@...tuousgeek.org, yinghai@...nel.org,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] PCI: Always set prefetchable base/limit upper32
 registers

On Mon, 2009-11-30 at 17:19 -0700, Grant Grundler wrote:
> On Mon, Nov 30, 2009 at 05:03:32PM -0700, Grant Grundler wrote:
> > On Mon, Nov 30, 2009 at 02:51:44PM -0700, Alex Williamson wrote:
> > > Prior to 1f82de10 we always initialized the upper 32bits of the
> > > prefetchable memory window, regardless of the address range used.
> > > Now we only touch it for a >32bit address, which means the upper32
> > > registers remain whatever the BIOS initialized them too.
> > > 
> > > It's valid for the BIOS to set the upper32 base/limit to
> > > 0xffffffff/0x00000000, which makes us program prefetchable ranges
> > > like 0xffffffffabc00000 - 0x00000000abc00000
> > > 
> > > Revert the chunk of 1f82de10 that made this conditional so we always
> > > write the upper32 registers and remove now unused pref_mem64 variable.
> > > 
> > > Signed-off-by: Alex Williamson <alex.williamson@...com>
> > 
> > Reviewed-by: Grant Grundler <grundler@...isc-linux.org>
> 
> NAK this - I messed up. Yinghai is correct. Something else is going on.
> 
> It might be perfectly OK to read 0xffffffffabc00000 if the bridge
> isn't using the upper32 Prefetchable register. Maybe the problem is
> some code is reading the upper32 value without checking that it's valid?

Apologies for not threading the v2 patch into the original thread.  The
prefetchable base register does support the upper32 bits and it does
work correctly.  However per the pci-to-pci bridge spec, a little lower
on page 47, devices only supporting 32bit prefetchable ranges are to
implement the upper32 registers as read-only registers that return zero.
In the example above, -1 in the upper32 base simply means that base >
limit, which disables the range.

Further investigation shows that the MEM_64 resource flag is setup for
this range based on hardware capabilities, but then it gets removed in
pbus_size_mem() because we want to use the range to map a 32bit option
ROM.  This leaves us entering pci_setup_bridge() with -1 in the upper32
base and the MEM_64 flag clear, so we never touch the upper32 base
register.  I think this patch is still a simple, safe solution.  Thanks,

Alex

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