Re: Mapping virtual addresses to physical

[Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx>]

On Wed, Jul 02, 2008 at 11:01:50PM +0100, Alan Casey wrote:
> >On Wed, Jul 02, 2008 at 09:42:26PM +0100, Alan Casey wrote:
> >>   I have used code similar to the devmem2.c example at
> >>   www.simtec.co.uk/appnotes/AN0014/ to map physical
> >>   addresses to virtual addresses (i.e. user space pointers)
> >>   but am just wondering how do you do the reverse - i.e.
> >>   map virtual addresses to physical addresses in a user
> >>   space program without writing a device driver??
> >
> >What kind of interface to this functionality are you expecting?  What
> >use case would such an interface have?
> 
>   I need to get the virtual address of a file on a Linux filesystem

Hmm.  Not sure how to break this to you, but files on a Linux filesystem
don't have virtual addresses.  The only time that they live in memory
is when they're loaded into the page cache.

At any moment in the future, data in the page cache can be evicted.  So,
even if you could work out where a file was, it'd be entirely useless
because you can't guarantee (from userspace) that it'll stay there.

>   or a memory buffer and convert it to a physical address for the
>   purposes of programming a custom DMA Controller (on an FPGA) with
>   source read (+destination write) addresses.

It's far better to write a small device driver for your FPGA to do this.

Essentially, you want the device driver to provide a character special
device.

Your userspace program opens the device, and then calls mmap() on it to
map some memory into its address space.  In response, the device driver's
mmap method allocates the requested memory (using dma_alloc_coherent()) and
maps it into userspace using dma_mmap_coherent().

You could then have an ioctl method to say "start DMA" and "stop DMA"
which'd control the DMA operation.  Please ensure that you use numbers
defined using the macros in asm-generic/ioctl.h.

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