Re: Mapping virtual addresses to physical

["Alan Casey" <alan.casey5@xxxxxxxxxxx>]

>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.

 Thanks for the info - looks like i better start reading the Linux Device
 Drivers book, but if you have any links to good examples that have the
 functionality that i need, let me know please.

 Regards,
 Alan.



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