NETMAP and off-by-one?

[Slawa Olhovchenkov]

On Fri, Nov 13, 2015 at 03:40:04PM -0800, Luigi Rizzo wrote:

> On Fri, Nov 13, 2015 at 1:30 PM, Slawa Olhovchenkov <slw at zxy.spb.ru> wrote:
> > I am see strange things: like NETMAP stop transmit after `head` and `cur`
> > touch `tail`.
> >
> > But:
> >
> > /*
> >  * check if space is available in the ring.
> >  */
> > static inline int
> > nm_ring_empty(struct netmap_ring *ring)
> > {
> >         return (ring->cur == ring->tail);
> > }
> >
> > i.e. if cur == (tail-1) mod ring_size -- space is available in the
> > ring and I can put packet in output buffer.
> 
> The design requires to leave at least one empty slot in the buffer.
> The name of the function is correct, the comment is probably not,
> unless a bug has creeped in recently the code was very careful
> in not using the free slot that separates the two regions.

Please, do some clarification, in case for transmit patch:

- can I put in txring in case cur == (tail-1) mod ring_size?
- can I use poll with only `events |= POLLIN` for transmiting?

in case for receive patch:

- can I detect input overflow?

> cheers
> luigi
> 
> > After put this packet ring is full, but no transmiting.
> >
> > This is bug?
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> 
> 
> 
> -- 
> -----------------------------------------+-------------------------------
>  Prof. Luigi RIZZO, rizzo at iet.unipi.it  . Dip. di Ing. dell'Informazione
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