Re: Map PCI resource to user space problem.

From: Armin Schindler
Date: Mon Jun 20 2011 - 12:38:25 EST

On Mon, 20 Jun 2011, Armin Schindler wrote:

when I try to map PCI resource of size < PAGE_SIZE to user space
with e.g. remap_pfn_range() the mapping seems to be created,
but the access to the PCI device just doesn't work (read returns 0xff).

The same happens when using sysfs. The device has
# ls -al /sys/bus/pci/devices/0000\:02\:09.0/resource*
-rw------- 1 root root 512 Jun 20 11:33 /sys/bus/pci/devices/0000:02:09.0/resource0
-rw------- 1 root root 256 Jun 20 11:33 /sys/bus/pci/devices/0000:02:09.0/resource1
-rw------- 1 root root 8388608 Jun 20 11:33 /sys/bus/pci/devices/0000:02:09.0/resource2
-rw------- 1 root root 262144 Jun 20 11:33 /sys/bus/pci/devices/0000:02:09.0/resource3

and when I mmap() resource0, a valid address is returned, but reading the area always returns 0xff.

Since a kernel driver can access resource0 here without problems when using
ioremap(), I'm not sure what is wrong with the sysfs entry for user space
(or using remap_pfn_range() in own mmap function).
The only hint I have so far is the size, which is smaller than PAGE_SIZE.

I found the reason. It is not the size of the resource.
The resource0 is assigned to a not page-aligned address.
Example, the pci resource0 has address 0xfe5ffc00. mmap()ing it will
actually seems to map address 0xfe5ff000 and the user needs to add 0xc00 to the address returned by mmap().


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