linux-kernel - Re: [PATCH 2/3] PCI: Add quirks for devices found on Cavium ThunderX SoCs.

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Date:	Fri, 18 Sep 2015 21:45:50 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	David Daney <ddaney@...iumnetworks.com>
Cc:	linux-arm-kernel@...ts.infradead.org,
	David Daney <ddaney.cavm@...il.com>,
	linux-kernel@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
	linux-pci@...r.kernel.org, Will Deacon <will.deacon@....com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>, devicetree@...r.kernel.org,
	Marc Zyngier <marc.zyngier@....com>,
	David Daney <david.daney@...ium.com>
Subject: Re: [PATCH 2/3] PCI: Add quirks for devices found on Cavium ThunderX SoCs.

On Friday 18 September 2015 10:00:32 David Daney wrote:
> On 09/18/2015 12:19 AM, Arnd Bergmann wrote:
> > On Thursday 17 September 2015 15:41:33 David Daney wrote:
> >> From: David Daney <david.daney@...ium.com>
> >>
> >> The on-chip devices all have fixed bars.  So, fix them up.
> >>
> >> Signed-off-by: David Daney <david.daney@...ium.com>
> >>
> >
> > You should be able to just mark the BARs as fixed in DT
> 
> In the case of ACPI, there is no DT.  So we would need a different 
> solution for ACPI.  What would you recommend for ACPI?

I would expect that this does not matter at all on ACPI, because
the devices that need it are not hot-plugged, and all boot-time
devices are probed by the firmware: the ACPI PCI implementation
does not reassign any BARs, except for the hotplug case.

> Also, can you point me to the OF device tree specification where it 
> tells how to specify PCI BAR addresses, I would especially be interested 
> in knowing how to specify fixed SRIOV BAR addresses in the device tree.

This is the 'n' bit mentioned sections 2.1.3 and 2.2.1.1 of the
PCI binding. When it is set, the OS is not supposed to try to
reassign the BAR even on machines that otherwise do a complete
rescan.

The PCI binding traditionally requires you to list all PCI devices
in DT, Linux as an extension (for the flattened DT format) allows
leaving out the devices, but in this case you probably need to
list every device that has a fixed BAR.

> Yes, it is a bit of a hack.  That is why I put it in its own file, and 
> only try to hack up PCI devices that exactly match the vendor and device 
> ids that need fixing.
> 
> IMHO, putting infrastructure into drivers/pci/probe.c, et al. to handle 
> this would be much more intrusive.

My guess is that it's already there, but even if it's not, this is a
generic well-defined case that has a standardized binding, and we should
implement that.

> For the record:  The PCI Enhanced Allocation (EA) capability (approved 
> by PCI SIG on 23 October 2014) is the proper way to handle this going 
> forward.  However, this is not yet implemented in the SoCs that this 
> patch addresses.  Our plan is to implement the EA capability in the core 
> PCI code, so that we do not need to keep adding devices to this fixup code.

Good, but still this should only be required for the embedded case where
you don't have a firmware to probe the bus.

	Arnd
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