On Freedag 27 Mai 2005 21:54, Timur Tabi wrote:
> Christoph Hellwig wrote:
>
> > It will return the correct physical address for the start of the buffer.
No, not even that. If you do __pa(vmalloc()), the result will point outside of
the physical address space on most architectures.
> > But given that vmalloc is a non-contingous allocator that's pretty useless.
>
> So as long as the vmalloc'd memory fits inside one page, __pa() will always give the
> correct address? If so, then can't I just call __pa() for every page in the buffer and
> get a list of physical addresses? If I can do that, then how the memory be virtually
> contiguous but not physicall contiguous?
If the vmalloc'd memory fits into one page, you should not have used
vmalloc in the first place ;-). The only reason to ever use vmalloc
is if you can't get enough memory from alloc_pages reliably.
> > As are physical addresses for anything but low-level architecture code.
>
> I don't understand what that means.
It means that a device driver should never need to use __pa directly, because
physical addresses don't have a well-defined meaning outside of the memory
management. A driver should only need to deal with user virtual, kernel virtual
and bus virtual but never real addresses.
Also, no device driver should be using vmalloc either: vmalloc fragments
your address space, pins physical pages and eats small children.
Arnd <><
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