Re: [PATCH] New phys_addr() syscall

Alan Cox (alan@lxorguk.ukuu.org.uk)
Mon, 20 Jul 1998 00:27:44 +0100 (BST)

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> 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.

Alan

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