On Sep 1, 2013, at 06:16, Bryan Venteicher <bry...@freebsd.org> wrote: > Author: bryanv > Date: Sun Sep 1 04:16:43 2013 > New Revision: 255109 > URL: http://svnweb.freebsd.org/changeset/base/255109 > > Log: > Add support for postponing VirtIO virtqueue interrupts > > Partial support for the EVENT_IDX feature was added a while ago, > but this commit adds an interface for the device driver to hint > how long (in terms of descriptors) the next interrupt should be > delayed. > > The first user of this will be used to reduce VirtIO net's Tx > completion interrupts. ... > int > -virtqueue_postpone_intr(struct virtqueue *vq) > +virtqueue_postpone_intr(struct virtqueue *vq, vq_postpone_t hint) > { > uint16_t ndesc, avail_idx; > > - /* > - * Request the next interrupt be postponed until at least half > - * of the available descriptors have been consumed. > - */ > avail_idx = vq->vq_ring.avail->idx; > - ndesc = (uint16_t)(avail_idx - vq->vq_used_cons_idx) / 2; > + ndesc = (uint16_t)(avail_idx - vq->vq_used_cons_idx); > + > + switch (hint) { > + case VQ_POSTPONE_SHORT: > + ndesc /= 4; > + break; > + case VQ_POSTPONE_LONG: > + ndesc *= 3 / 4; > + break;
The compiler will fold the "3 / 4" expression to zero, so ndesc will
always end up as zero because of this. I assume that what you want is
this instead:
ndesc = (ndesc * 3) / 4;
or if you want rounding:
ndesc = (ndesc * 3 + 2) / 4;
-Dimitry
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