linux-kernel - Re: [PATCH RFC 2/2] arm: kernel: fix pci_mmap_page

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Date:	Thu, 16 Oct 2014 11:24:45 +0100
From:	Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>,
	LAKML <linux-arm-kernel@...ts.infradead.org>,
	Linux PCI <linux-pci@...r.kernel.org>
Subject: Re: [PATCH RFC 2/2] arm: kernel: fix pci_mmap_page_range() offset
 calculation

Hi Russell,

thanks for having a look.

On Wed, Oct 15, 2014 at 11:29:32PM +0100, Russell King - ARM Linux wrote:
> On Wed, Oct 15, 2014 at 01:03:41PM +0100, Lorenzo Pieralisi wrote:
> > ARM relies on the standard implementation of pci_resource_to_user()
> > which actually is an identity map and exports to user space
> > PCI memory resources as they are stored in PCI devices resources (ie BARs)
> > which represent CPU physical addresses (fixed-up using BUS to CPU
> > address conversions) not PCI bus addresses.
> 
> This paragraph seems wrong.
> 
> It first says that PCI memory resources contain the same values that the
> PCI device has in its BAR.  It then goes on to say that they are CPU
> physical addresses.  That is not true.
> 
> For example, DC21285 systems always have done this as: the PCI bars
> contain the _bus_ addresses, which tend to be in the range 0 to
> 0x7fffffff.  These correspond with a CPU physical address of
> 0x80000000 to 0xffffffff.  The PCI bus resources for IOMEM resources
> contains the CPU physical address of the mapping.

It is a commit log wording problem, I exactly meant what you said, I
will reword it (or remove "ie BARs" from it, since it is misleading).

I think that the word "BAR" is a bit misused in helpers function like:

pci_resource_{start/end/len}

too but as long as we all know what that means (and I write proper
commit logs :)) it is all fine.

> > On platforms where the mapping between CPU and BUS address is not a 1:1
> > mapping this is erroneous, in that an additional shift is applied to
> > an already fixed-up offset passed from userspace.
> 
> Yes, I think this is a correct patch inspite of the description. :)

Great, I will reword it and wait for comments on patch 1 that changes
pci_mmap_fits() (it does not affect ARM, but would like to get both changes
in coherently - ie if I am asked to change patch 1 I will probably have
to change this patch too).

Thanks,
Lorenzo

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