linux-kernel - Re: [RFC/RFT PATCH v2] PCI: move pci_read_bridge

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Date:	Sat, 13 Jun 2015 10:12:00 +0100
From:	Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Ralf Baechle <ralf@...ux-mips.org>,
	"James E.J. Bottomley" <jejb@...isc-linux.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Richard Henderson <rth@...ddle.net>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Howells <dhowells@...hat.com>,
	Russell King <linux@....linux.org.uk>,
	Tony Luck <tony.luck@...el.com>,
	"David S. Miller" <davem@...emloft.net>,
	Ingo Molnar <mingo@...hat.com>,
	Michal Simek <monstr@...str.eu>,
	Chris Zankel <chris@...kel.net>, Arnd Bergmann <arnd@...db.de>,
	Krzysztof Halasa <khalasa@...p.pl>,
	Phil Edworthy <phil.edworthy@...esas.com>,
	Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
	Jingoo Han <jg1.han@...sung.com>,
	Lucas Stach <l.stach@...gutronix.de>,
	Simon Horman <horms@...ge.net.au>,
	Minghuan Lian <minghuan.Lian@...escale.com>,
	Murali Karicheri <m-karicheri2@...com>,
	Tanmay Inamdar <tinamdar@....com>,
	Kishon Vijay Abraham I <kishon@...com>,
	Thierry Reding <thierry.reding@...il.com>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Will Deacon <Will.Deacon@....com>,
	Jayachandran C <jchandra@...adcom.com>,
	"suravee.suthikulpanit@....com" <suravee.suthikulpanit@....com>
Subject: Re: [RFC/RFT PATCH v2] PCI: move pci_read_bridge_bases to the
 generic PCI layer

On Sat, Jun 13, 2015 at 02:47:55AM +0100, Guenter Roeck wrote:
> On Tue, Jun 09, 2015 at 10:01:45AM +0100, Lorenzo Pieralisi wrote:
> > When a PCI bus is scanned, upon PCI bridge detection the kernel
> > has to read the bridge registers to set-up its resources so that
> > the PCI resource hierarchy can be validated properly.
> > 
> > Most if not all architectures read PCI bridge registers in the
> > pcibios_fixup_bus hook, that is called by the PCI generic layer
> > whenever a PCI bus is scanned.
> > 
> > Since pci_read_bridge_bases is an arch agnostic operation (and it
> > is carried out on all architectures) it can be moved to the generic
> > PCI layer in order to consolidate code and remove the respective
> > calls from the architectures back-ends.
> > 
> > The PCI_PROBE_ONLY flag is not checked before calling
> > pci_read_bridge_buses in the generic layer since reading the bridge
> > bases is not related to resources assignment; this implies that it
> > can be carried out safely on PCI_PROBE_ONLY systems too and should
> > not affect architectures (alpha, mips) that check the PCI_PROBE_ONLY
> > flag before reading the bridge bases.
> > 
> > In order to validate the resource hierarchy as soon as the resources
> > themselves are probed (ie read from the bridge), this patch also adds
> > code to pci_read_bridge_bases that claims the bridge resources, so that
> > they are validated and inserted in the resource hierarchy as soon as
> > the bridge bases are probed.
> > 
> 
> Hi Lorenzo,
> 
> on one of our systems, I see a lot of messages with your patch applied.
> 
> bart kernel: pci 0000:b0:00.0: can't claim BAR 7 [io 0x0000-0x0fff]: no compatible bridge window
> bart kernel: pci 0000:b0:00.0: can't claim BAR 8 [mem 0x94000000-0x941fffff]: no compatible bridge window
> bart kernel: pci 0000:b1:03.0: can't claim BAR 7 [io 0x0000-0x0fff]: no compatible bridge window
> bart kernel: pci 0000:b1:03.0: can't claim BAR 8 [mem 0x95800000-0x959fffff]: no compatible bridge window
> 
> and so on. The final IO memory assignment is the same, though,
> before and after your patch.
> 
>       95800000-95bfffff : PCI Bus 0000:b0
>         95800000-959fffff : PCI Bus 0000:b1
> 	  95800000-959fffff : PCI Bus 0000:b2
> 	95a00000-95a3ffff : 0000:b0:00.0
> 
> 
> Does that have any relevance or is it just nuisance messages ?

Yes, I knew this could happen. It should be just nuisance messages,
since we are claming bridge resources even on systems where they
are reassigned. We should remove those messages, this means that I
have to craft a function that claims resources without spitting too
much unwanted noise, I can't use pci_claim_bridge_resource for this
purpose as you have noticed, unless I refactor it, open to suggestions
(we claim bridge resources by default, regardless of PROBE_ONLY flag).

Thanks a lot for testing it, appreciated, I will prepare a v3.

Lorenzo

> 
> Thanks,
> Guenter
> 
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