Re: [PATCH] "volatile considered harmful" document

["H. Peter Anvin" <hpa@zytor.com>]

Jonathan Corbet wrote:
> OK, here's an updated version of the volatile document - as a plain text
> file this time.  It drops a new file in Documentation/, but might it be
> better as an addition to CodingStyle?
> 
> Comments welcome,

I have found one use of volatile which I consider legitimate: pointers
to data structures read or written by I/O devices in coherent memory
(typically pci_coherent memory.)  This is local to device drivers, but
as far as I can tell, the use of volatile here is legitimate, although
arguably it will be redundant in > 90% of all cases due to the
incidental presence of other memory barriers.

In Ethernet drivers, for example, it is common for the network card to
maintain a pointer in host memory the the latest descriptor written; you
will generally have a loop of the form:

	while ((this_pointer = *pointer_ptr) > my_last_pointer) {
		for (pkt = my_last_pointer; pkt < this_pointer; pkt++)
			receeive_packet(pkt);
		my_last_pointer = this_pointer;
	}

pointer_p can then be a volatile pointer into said coherent memory.

	-hpa