Messages in this thread | | | Date | Wed, 09 Sep 2009 12:54:29 -0600 | From | "Chris Friesen" <> | Subject | Re: [PATCH] x86: SGU UV Add volatile to macros that access chipset registers |
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On 09/09/2009 12:11 PM, Daniel Walker wrote: > On Wed, 2009-09-09 at 13:01 -0500, Jack Steiner wrote: >> Volatile is being added to the accessor functions that are used to >> read/write memory-mapped I/O registers located within the UV chipset. >> The use of volatile is hidden within the functions and is not exposed >> to the users of the functions. >> >> Note that the use is limited to the accessor functions in the header >> file. No .c files are changed or need to know about volatile. >> >> >> This seems to be consistent with other uses of volatile within the kernel. > > The document that I cited specifically addresses memory accessors as not > needing the volatile keyword .. So your still not addressing exactly why > your code needs it .. Are your accessors special in some way? Is there > some defect your seeing without the volatile keyword?
From that document:
"There are still a few rare situations where volatile makes sense in the kernel:
- The above-mentioned accessor functions might use volatile on architectures where direct I/O memory access does work. Essentially, each accessor call becomes a little critical section on its own and ensures that the access happens as expected by the programmer."
However, the fact that these functions are returning volatile pointers is a bit sketchy. Normally accesses to such memory would be wrapped entirely in a read/write function which would handle the volatility internally to the function.
In this case there's no point in the function returning a "volatile unsigned long *" if it's going to be cast to a volatile pointer before being dereferenced.
Chris
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