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On 11/03/14 15:44, Ian Campbell wrote:
On Thu, 2014-03-06 at 15:47 +0000, Zoltan Kiss wrote:

I was wondering for a while why this macro looks like this:

#define RING_HAS_UNCONSUMED_REQUESTS(_r)                                \
     ({                                                                 \
        unsigned int req = (_r)->sring->req_prod - (_r)->req_cons;     \
        unsigned int rsp = RING_SIZE(_r) -                              \
                           ((_r)->req_cons - (_r)->rsp_prod_pvt); \
        req < rsp ? req : rsp;                                               \

I would expect to check prod - cons, like RING_HAS_UNCONSUMED_RESPONSES does:

#define RING_HAS_UNCONSUMED_RESPONSES(_r)                               \
     ((_r)->sring->rsp_prod - (_r)->rsp_cons)

By my understanding, there is no way rsp could be smaller than req, so
there is no point having this. Am I missing something?

Just looking at this again. All that stuff I said about wraparound was
misleading/irrelevant since req and rsp are not the pointers, but
actually the number of things. Sorry.

Is it the case that this macro considers a request to be unconsumed if
the *response* to a request is outstanding as well as if the request
itself is still on the ring?
I don't think that would make sense. I think everywhere where this macro is called the caller is not interested in pending request (pending means consumed but not responded)

I wonder if this apparently weird construction is due to pathological
cases when one or the other end is not picking up requests/responses?
i.e. trying to avoid deadlocking the ring or generating an interrupt
storm when the ring it is full of one or the other or something along
those lines?

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