Re: Problem mapping small PCI memory space.

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Thu, 2005-05-19 at 15:43 -0400, Richard B. Johnson wrote:
> 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.

There are a number of totally bogus statements above.

First of all, you don't have to implement mmap in your driver (though it
would probably better to do so). You can perfectly map your PCI memory
space from userland without help of a specific driver (see below).

Then, ioremap has absolutely nothing to do with the mapping done to
userland (unless there is some gory x86 in there, but I really doubt
it). ioremap_* has to do with mapping the memory in kernel space, not
user space. Thus your statement "must be ioremap_nocache" is wrong.

I don't know why you think the pages should be reserved as well, damn,
he's not mapping RAM, he's mapping PCI MMIO space, do you know what you
are talking about at all ? There is usually no struct page for PCI MMIO
space.
 
> 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.

Oh well, Ginancula, here is what you can do:

 - If you want to stick to /dev/mem, which is not recommended and may
not work on all platforms but is fine for a quick hack, just make sure
you are mmap'ing a full page. That is, your 512 bytes will be
512-bytes-aligned somewhere in a page of 4k on most platforms, maybe
more. You have to map this entire page, and then take an offset in the
resulting mapping to the 512 bytes.
Basically, you use getpagesize() to get the page size, then you mmap
your base_addr & ~(page_size - 1), and you access your data at the
resulting address + base_addr & (page_size - 1)

 - Best is to use the PCI mmap facility. The "old" one via /proc/bus/pci
or the "new one" via /sys/...path to your device.../resourceN. You
directly mmap the resource file you want. HOWEVER, the alignement
restriction will still be there, thus you should also look at the
"resource" file to get the actual base address, and deduce the
alignement, and then map a whole page and add the alignement offset to
the result as for /dev/mem.

Ben.


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