linux-kernel - Re: [PATCH] PCI: Clear bridge MEM_64 flag if one child does not support it

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Date:	Tue, 09 Dec 2014 10:00:03 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Bjorn Helgaas <bhelgaas@...gle.com>,
	Gavin Shan <gwshan@...ux.vnet.ibm.com>,
	Marek Kordík <kordikmarek@...il.com>,
	zermond@...il.com, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] PCI: Clear bridge MEM_64 flag if one child does not
 support it

On Tue, 2014-12-09 at 09:56 +1100, Benjamin Herrenschmidt wrote:
> On Mon, 2014-12-08 at 13:52 -0800, Yinghai Lu wrote:
> > 2. or scan the children resource other than ROM to clear bridge MEM_64
> >    for mmio pref.
> > 
> > The patch is using second way so will keep child mmio pref into bridge
> > mmio pref range.
> 
> That means that having a single ROM BAR that is 32-bit and prefetchable
> will downgrade the entire window to 32-bit ? That's not going to work
> either.
> 
> I have GPUs with 16G BARs for example... suddenly they don't fit
> anaymore because we downgraded the window to 32 bit because somewhere
> there's a 32-bit pref resource ?
> 
> That will break more than it fixes...

I think an option would be to keep track of whether the PCI host bridge
can do 64-bit pref above 4G.

If it can't then we know 64-bit pref is always fair game for 32-bit
resources and we can thus downgrade all the bridges 64-pref to 32-perf.

If it can, then we probably do need to leave it there do real 64-bit
pref, and shoot anything that is 32-bit only down the non-pref windows.

We *could* try to be smart and scan first to check if anything under the
bridge actually has large/64-bit BARs but that's going to be a heuristic
at best and isn't going to do any good with hotplug.

I tend to think that treating anything 32-bit pref as non-pref is in
fact the best solution unless we know that the platform doesn't do
>32-bit pref anyway in which case we leave them alone.

Ben.


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