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

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Message-ID: <20130109212847.GT3931@n2100.arm.linux.org.uk>
Date:	Wed, 9 Jan 2013 21:28:47 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
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>,
	Stephen Warren <swarren@...dotorg.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Andrew Murray <andrew.murray@....com>,
	Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
	Arnd Bergmann <arnd@...db.de>,
	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 Wed, Jan 09, 2013 at 09:43:05PM +0100, Thierry Reding wrote:
> The I/O map cache is used to map large regions of physical memory in
> smaller chunks to avoid running out of vmalloc()/ioremap() space.
> 
> Signed-off-by: Thierry Reding <thierry.reding@...onic-design.de>

We already have a means where we record the mappings which ioremap()
creates.  If you look at /proc/vmallocinfo, you'll notice lines such
as:

0xf7b72000-0xf7b74000    8192 e1000_probe+0x291/0xa68 [e1000e] phys=fc025000 ioremap

which gives you the virtual address range, physical address and type
of the mapping.  Why do we need a duplicated data structure?

Moreover, you seem to suggest that you want to break up a large
ioremap() mapping into several smaller mappings.  Why?  The idea
behind ioremap() is that this relationship holds true:

	ptr = ioremap(cookie + n, size);

For any 'n' in the range 0 .. size, the location shall be accessible
via ptr + n when using the IO space accessors.  If you're going to
break up a mapping into several smaller ones, this no longer holds
true.

If the problem is that you're ioremapping huge address ranges because
you're passing larger-than-required resources to devices, then that's
part of the problem too.
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