On Wed, Jun 16, 2010 at 08:53:57PM -0700, Michael Chan wrote:
> Mike Frysinger wrote:
>
> > On Wed, Jun 16, 2010 at 9:13 PM, James Bottomley wrote:
> > > I'm not quite sure whose fault this one is.
> > >
> > > However, this code in bnx2.c:
> > >
> > > if (!get_dma_ops(&pdev->dev)->sync_single_for_cpu) {
> > > next_rx_buf =
> > > &rxr->rx_buf_ring[
> > > RX_RING_IDX(NEXT_RX_BD(sw_cons))];
> > > prefetch(next_rx_buf->desc);
> > > }
> > >
> > > Looks remarkably fragile: what exactly is it trying to do?
>
> If sync_single is not defined, that means the CPU has a consistent
> view of next_rx_buf and so it makes sense to prefetch it.
>
Except that's not a valid assertion, there are platforms that implement
it for sanity checks yet still have consistent DMA. You are making
inherently non-portable assumptions for a PCI driver, which is a good
example of why drivers should never be side-stepping the API in the first
place. If you want to have a micro-optimization for the consistent DMA
case, you can check dma_is_consistent(), which is part of the API and
will be variable on certain platform configurations (ie, some may be
consistent with PCI but not on other busses, etc.)
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