linux-kernel - Re: [PATCH v2 2/4] PCI: pci-host-generic: Fix lookup of linux,pci-probe-only property

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Date:	Mon, 17 Aug 2015 10:01:32 +0100
From:	Will Deacon <will.deacon@....com>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	Marc Zyngier <Marc.Zyngier@....com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	"suravee.suthikulpanit@....com" <suravee.suthikulpanit@....com>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	Rob Herring <robh+dt@...nel.org>,
	"agraf@...e.de" <agraf@...e.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH v2 2/4] PCI: pci-host-generic: Fix lookup of
 linux,pci-probe-only property

On Fri, Aug 14, 2015 at 09:26:21PM +0100, Bjorn Helgaas wrote:
> On Fri, Aug 14, 2015 at 11:43 AM, Will Deacon <will.deacon@....com> wrote:
> > On Fri, Aug 14, 2015 at 05:40:51PM +0100, Bjorn Helgaas wrote:
> >> Do we need support for pci-probe-only in pci-host-generic at all?
> >> You're removing the use in amd-overdrive.dts, and there are no other
> >> DTs in the kernel tree that mention it.
> >>
> >> If we can live without it, that would be nice.  It seems like a relic from
> >> days when we couldn't reliably assign resources.  (I'm not saying we can do
> >> that reliably even today, but I'd rather make it reliable than turn it
> >> off.)
> >
> > Kvmtool certainly uses it (and generates its own DT, hence why you don't
> > see it in mainline). Not sure about qemu, though.
> 
> Do you know why kvmtool wants probe-only?  Would something break if we
> didn't support probe-only?  I guess we'd be looking for a case where
> Linux assigns resources and that assignment doesn't work with kvmtool?

It's basically because the BARs aren't writable other than to find the
region size. It could fixed with a bit of pain, but it doesn't help older
kvmtools that do work with mainline today.

Will
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