Re: Problem mapping small PCI memory space.

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----- Original Message ----- From: "Richard B. Johnson" <[email protected]>
To: "Gianluca Varenni" <[email protected]>
Cc: "Linux kernel" <[email protected]>
Sent: Thursday, May 19, 2005 12:43 PM
Subject: Re: Problem mapping small PCI memory space.


On Thu, 19 May 2005, Gianluca Varenni wrote:

Hi all.

I'm writing a driver for a PCI board that exposes two memory spaces (out of
the 6 IO address regions).

One of them is 1MB, and I can map it to user level without problems. The
other one is only 512 bytes.
If I try to open it with /dev/mem, it returns EINVAL (the 1MB memory space
is opened without any problem). If I try to expose it through mmap, mmap
succeeds, but I only see garbage at user level. At kernel level, I can
access that 512 bytes memory by using ioremap() on the physical address
returned by pci_resource_start().

Are there any lower limits on the size of a PCI memory region?

Have a nice day
GV


You impliment mmap() in your driver. It accesses the first megabyte
as 256 pages. Then you tack on the additional page that your 512
bytes resides at. mmap() only works with pages. The pages must
be ioremap_nocache and they must be reserved. The reserved part
is important to have them visible from user-space using mmap.

That's IFF you really need to see the stuff in user-mode. Normally,
you write a driver that accesses everything using the PCI primatives
provided in the kernel, for the kernel.

Do you have any link to a linux driver working exactly like this?

Have a nice day
GV



Cheers,
Dick Johnson
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