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

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Message-Id: <201301092210.49452.arnd@arndb.de>
Date:	Wed, 9 Jan 2013 22:10:49 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	Thierry Reding <thierry.reding@...onic-design.de>
Cc:	linux-tegra@...r.kernel.org,
	Grant Likely <grant.likely@...retlab.ca>,
	Rob Herring <rob.herring@...xeda.com>,
	Russell King <linux@....linux.org.uk>,
	Stephen Warren <swarren@...dotorg.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Andrew Murray <andrew.murray@....com>,
	Jason Gunthorpe <jgunthorpe@...idianresearch.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 Wednesday 09 January 2013, Thierry Reding wrote:
> What happens on Tegra is that we need to map 256 MiB of physical memory
> to access all the PCIe extended configuration space. However, ioremap()
> on such a large region fails if not enough vmalloc() space is available.
> 
> This was observed when somebody tested this on CardHu which has a 1 GiB
> of RAM and therefore remapping the full 256 MiB fails.

Hmm, config space accesses are fairly rare and generally not expected
to be fast, and 256 MB is really a huge waste of virtual address space,
so I agree that just ioremapping the entire space is not a good
solution.

However, it's not clear that a cache is necessary. Have you measured
a significant performance benefit of this implementation over just
iorempping and unmapping a single page for every config space access?
Have you checked if the hardware supports an alternative config
space access mechanism that does not depend on a huge address range?
A lot of them provide an index/data register pair somewhere, as the
original PC implementation did.

Even if we actually want a cache, how about a private implementation
that just remembers a single page in LRU? I doubt that there are
more drivers that would benefit from a generalized version that you
provide.

	Arnd
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