Question on rx queue in ixgbe driver

[Xu Zhe]

Hi,

I am reading ixgbe driver of Freebsd and got some problems, which are
described below.

(1) Why rxd_tail does not equals to rxd_head?

Here, rxd_tail/rxd_head is the value of
dev.ix.0.queue0.rxd_tail/dev.ix.0.queue0.rxd_head from sysctl (take the first
queue of ix0 as example).

Actually, in most cases, rxd_head - rxd_tail == 1.

Refers to the code, these values are actually next_to_refresh/next_to_check
for each receive queue (though the sysctl implementation is read directly from
the hardware registers I suppose). Why next_to_refresh is always one smaller
than next_to_check (of course, when the latter is 0, the former is 2047
possibly, which is the size of rx ring - 1)? In my point of view, this means
that we will always have one tiny mbuf (which is pointed by next_to_refresh)
that is already checked (passed up to upper network stack) but not refreshed
(not prepared for the next receive). It does not make sense? Or I missed
anything important?

(2) The init value of rxd_tail

Even if (1) has no problem, when ixgbe device is inited, rxd_tail (or say,
next_to_refresh) is set to zero (in function ixgbe_setup_receive_ring). I
think it should be rxr->num_desc - 1.

This should not matter much in the latest ixgbe driver, but it might cause old
driver (ixgbe 2.5.8 at least) to double init the rx_ring descriptors (both in
ixgbe_setup_receive_ring when ixgbe init up, and the first entry of
ixgbe_refresh_mbufs of the first interrupt come). This is a case I met in my
test environment.

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Looking forward to any of your replies to help clarify my thoughts. Thanks in
advance.

Peter