On Tue, Sep 01, 2020 at 07:16:27PM +0200, Christoph Hellwig wrote:

Well, if IP22 doesn't speculate (which I'm pretty sure is the case),
dma_sync_single_for_cpu should indeeed be a no-op.  But then there
also shouldn't be anything in the cache, as the previous
dma_sync_single_for_device should have invalidated it.  So it seems like
we are missing one (or more) ownership transfers to the device.  I'll
try to look at the the ownership management in a little more detail
tomorrow.

this is the problem:

       /* Always check for received packets. */
        sgiseeq_rx(dev, sp, hregs, sregs);

so the driver will look at the rx descriptor on every interrupt, so
we cache the rx descriptor on the first interrupt and if there was
$no rx packet, we will only see it, if cache line gets flushed for
some other reason. kick_tx() does a busy loop checking tx descriptors,
with just sync_desc_cpu...

Thomas.

-- 
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea.                                                [ RFC1925, 2.3 ]