linux-kernel - Re: [RFT PATCH v3] PCI: move pci_read_bridge

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Message-ID: <55B11AB0.7050002@roeck-us.net>
Date:	Thu, 23 Jul 2015 09:47:44 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Bjorn Helgaas <bhelgaas@...gle.com>,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>
CC:	linux-kernel@...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>,
	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 <Suravee.Suthikulpanit@....com>
Subject: Re: [RFT PATCH v3] PCI: move pci_read_bridge_bases to the generic
 PCI layer

On 07/23/2015 09:12 AM, Bjorn Helgaas wrote:
> On Thu, Jul 09, 2015 at 11:59:16AM +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.
>>
>> Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@....com>
>
> Applied to pci/resource for v4.3, thanks!
>
> The PCI_PROBE_ONLY text seems backward to me: previously alpha and mips
> only called pci_read_bridge_bases() if PCI_PROBE_ONLY was set.  After this
> patch, alpha and mips systems that do not set PCI_PROBE_ONLY will also call
> pci_read_bridge_bases().
>
> I really don't know why alpha and mips were like that.  It seems backwards.
> It seems like we'd want to know the bridge windows if we were assigning
> things, but they only read them if they were *not* going to assign things.
>
> I'm a little uneasy that we might break some alpha or mips system, since
> there must have been some reason this was done originally.  It'd be ideal
> if somebody could test a non-PCI_PROBE_ONLY system.  But maybe they're all
> obsolete.
>

For alpha, PCI_PROBE_ONLY is set for two platforms, marvel and titan,
out of ~20. mips sets the flag for 6 platforms out of >25.
Unlikely that those are the only relevant ones.

I could try to run some qemu tests for both architectures, but I have
no idea what to look out for. Ideas, anyone ?

Guenter

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