Messages in this thread | | | Date | Thu, 10 May 2007 12:35:50 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH] "volatile considered harmful" document |
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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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