[openib-general] understanding mthca_alloc_db()

[Roland Dreier]

    yipee> Hi, I'm trying to understand what's going on inside
    yipee> mthca_alloc_db() (in libmthca/ src/memfree.c).  I
    yipee> understand that this function manages a bitmap-based
    yipee> freelist of doorbells for send, receive & completion
    yipee> queues.  Is there a possibility of a memory leak when
    yipee> posix_memalign() steps on a previously allocated
    yipee> 'db_tab->page[i].db_rec' (when all of 'db_tab->page[i].
    yipee> free[j]' is cleared)? isn't 'i' already out of bounds?

I don't think so.  i is being set by the loop

	for (i = start; i != end; i += dir)

so if we fall through the end of loop, then i == end, which is where
we want to allocate a new doorbell page.

    yipee> The functions defines two types/groups of doorbells. Why
    yipee> are these doorbells allocated differently (one group starts
    yipee> at the begining of the array and the other at the end)?

This is the way the hardware works.

    yipee> Another thing I noticed is that doorbells are different
    yipee> between Tavor and Arbel HCA's (e.g. see
    yipee> update_cons_index(). Is it correct that Arbel doorbells are
    yipee> only 32 bits wide?

Sort of.  It is definitely true that Tavor-mode doorbells work
differently from Arbel/mem-free-mode doorbells.

 - R.