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
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