linux-kernel - Re: [PATCH 05/14] lib: Add I/O map cache implementation

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Message-Id: <201301100917.19577.arnd@arndb.de>
Date:	Thu, 10 Jan 2013 09:17:19 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	Thierry Reding <thierry.reding@...onic-design.de>
Cc:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
	Stephen Warren <swarren@...dotorg.org>,
	linux-tegra@...r.kernel.org,
	Grant Likely <grant.likely@...retlab.ca>,
	Rob Herring <rob.herring@...xeda.com>,
	Russell King <linux@....linux.org.uk>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Andrew Murray <andrew.murray@....com>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	devicetree-discuss@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-pci@...r.kernel.org
Subject: Re: [PATCH 05/14] lib: Add I/O map cache implementation

On Thursday 10 January 2013, Thierry Reding wrote:
> On Wed, Jan 09, 2013 at 04:17:58PM -0700, Jason Gunthorpe wrote:
> > On Wed, Jan 09, 2013 at 04:12:31PM -0700, Stephen Warren wrote:
> > You could decrease the size of the mapping to only span the bus
> > numbers that are configured for use via DT.
> 
> That won't work, unfortunately. The mapping is such that the bus number
> is not encoded in the uppermost bits, the extended register number is.
> So the only thing that we could do is decrease the size of the extended
> register space for *all* devices.

But you could still a method to map 16 separate areas per bus, each 65536
bytes long, which results in 1MB per bus. That is probably ok, since
very few systems have more than a handful of buses in practice.

In theory, doing static mappings on a per-page base would let you
do 16 devices at a time, but it's probably worth doing at this fine
granularity.

> > Are there any concerns about these config registers being accessed
> > from a context where a new mapping can't be made? Interrupt? Machine
> > Check? PCI-E Advanced Error Reporting?
> 
> I haven't checked but I would expect configuration space accesses to not
> happen in interrupt context. Usually they are limited to enumeration and
> driver probe.

Actually, AER probably needs this, and I believe some broken devices 
need to mask interrupts using the PCI command word in the config space,
it it can happen.

	Arnd
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