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From: "Michael S. Tsirkin" <mst@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v2 2/2] e1000: decrement RDT if equal to RDH
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To: Jason Wang <jasowang@redhat.com>
Cc: Richard Tollerton <rich.tollerton@ni.com>, qemu-devel@nongnu.org, Jeff Westfahl <jeff.westfahl@ni.com>, kraxel@redhat.com

On Thu, Dec 18, 2014 at 12:01:48AM -0500, Jason Wang wrote:
> 
> 
> ----- Original Message -----
> > Some drivers set RDT=RDH. Oddly, this works on real hardware. To work
> > around this, autodecrement RDT when this happens.
> > 
> > Signed-off-by: Richard Tollerton <rich.tollerton@ni.com>
> > Signed-off-by: Jeff Westfahl <jeff.westfahl@ni.com>
> > ---
> >  hw/net/e1000.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> 
> Please describe more details on the issue.
> 
> The spec 3.2.6 said:
> 
> "
> When the head pointer is equal to the tail pointer, the ring is empty. 
> "
> 
> So RDT=RDH in fact empty the ring. No?

Richard, can you respond please?
I'd like to see this clarified in code comment or
commit message before applying this patchset.

> > 
> > diff --git a/hw/net/e1000.c b/hw/net/e1000.c
> > index 44ae3a8..b8cbfc1 100644
> > --- a/hw/net/e1000.c
> > +++ b/hw/net/e1000.c
> > @@ -1152,6 +1152,12 @@ mac_writereg(E1000State *s, int index, uint32_t val)
> >  static void
> >  set_rdt(E1000State *s, int index, uint32_t val)
> >  {
> > +    if (val == s->mac_reg[RDH]) {     /* Decrement RDT if it's too big */
> > +        if (val == 0) {
> > +            val = s->mac_reg[RDLEN] / sizeof(struct e1000_rx_desc);
> > +        }
> > +        val--;
> > +    }
> >      s->mac_reg[index] = val & 0xffff;
> >      if (e1000_has_rxbufs(s, 1)) {
> >          qemu_flush_queued_packets(qemu_get_queue(s->nic));
> > --
> > 2.1.3
> > 
> > 
> >