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SubjectRe: [PATCH] New phys_addr() syscall
Alan Cox writes:
> > Alan Cox writes:
> > > > People writing pure userspace network drivers may also find this handy
> > > > if they want to DMA directly into userspace.
> > >
> > > It doesnt help. You have locking and copy/write issues to worry about for
> > > the DMA case. This seems an extremely fringe and bizarre syscall to consider
> > > for general use!
> >
> > What issues? If I pin down a set of pages, I can then get their
> > physical address and tell the card to DMA its heart out. How exactly
> > is this going to fail?
>
> DMA doesnt honour the MMU write protect bits for copy on write, or for
> page locking. Nor does it set the dirty bit on page table entries.

OK, so it's not a safe and clean mechanism. On the other hand, if you
want a fast and dirty mechanism, it should work fine. Something akin
to the U-NET ATM drivers could use it.
Note: I'm not saying that this is the way networking should be done!
It's just something that I noticed you could do if you wanted.

Regards,

Richard....

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