On Wed, May 12, 2010 at 1:36 AM, David Howells <dhowells@xxxxxxxxxx> wrote:
>
> Out of interest, does it make the code smaller if you mark
> ioat2_get_ring_ent() and ioat2_ring_mask() with __attribute_const__?
>
> I'm not sure whether it'll affect how long gcc is willing to cache these, but
> once computed, I would guess they won't change within the calling function.
Unfortunately, it does not make a difference, but I'll keep this in
mind if ioat2_get_ring_ent() ever gets more complicated (which it
might in the future).
> Also, is the device you're driving watching the ring and its indices? If so,
> does it modify the indices? If that is the case, you might need to use
> read_barrier_depends() rather than smp_read_barrier_depends().
The device does not observe the indices directly. Instead we
increment a free running 'count' register by the distance between
ioat->pending and ioat->head.
>
>> + prefetch(ioat2_get_ring_ent(ioat, idx + i + 1));
>> + desc = ioat2_get_ring_ent(ioat, idx + i);
>> dump_desc_dbg(ioat, desc);
>> tx = &desc->txd;
>> if (tx->cookie) {
>
> Is this right, I wonder? You're prefetching [i+1] before reading [i]? Doesn't
> this mean that you might have to wait for [i+1] to be retrieved from RAM before
> [i] can be read? Should you instead read tx->cookie before issuing the
> prefetch? Admittedly, this is only likely to affect the reading of the head of
> the queue - subsequent reads in the same loop will, of course, have been
> prefetched.
Yes, it should be the other way around.
Thanks!
--
Dan
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