Hello,

On Wed, Nov 14, 2007 at 10:39:24PM +1100, Paul Mackerras wrote:
quoted text
> Christoph Hellwig writes:
> 
> >   int pfm_read_pmds(int fd, pfarg_pmd_t *pmds, int n)
> > 
> > This is basically a read(2) (or for other syscalls a write) on something
> > else than the file descriptor provided to the system call.
> 
> No it's not basically a read().  It's more like a request/reply
> interface, which a read()/write() interface doesn't handle very well.
> The request in this case is "tell me about this particular collection
> of PMDs" and the reply is the values.
> 

Exactly. This is not a brute force read()! On input you pass the list
of registers you want to read. Upon return, you get the list of values.

Now, I think the current call could be optimized even more by making
the structure smaller. Today, the structure passed read/write
PMD registers is the same. On write, we pass other information such as 
the reset values (sampling periods), randomization parameters and some
flags. They are not needed on read.

quoted text
> It seems to me that an important part of this is to be able to collect
> values from several PMDs at a single point in time, or at least an
> approximation to a single point in time.  So that means that you don't
> want a file per PMD either.
> 

Yes, we want to be able to read one or many registers in one call.
The number of PMU counters is not going to shrink, so having a file
descriptor per register looks overkill to me.

quoted text
> Basically we don't have a good abstraction for a request/reply (or
> command/response) type of interface, and this is a case where we need
> one.  Having a syscall that takes a struct containing the request and
> reply is as good a way as any, particularly for something that needs
> to be quick.
> 

-- 
-Stephane
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