Re: [dpdk-dev] [PATCH 1/6] ethdev: add descriptor status API

[Olivier Matz <olivier.matz@6wind.com>]

> >>> + * Check the status of a Rx descriptor in the queue  
> >> I think it would be useful to highlight caller context.
> >> Should it be the same CPU which receives packets from the queue?  
> > Yes, you are right it would be useful. I suggest the following sentences:
> >
> >    This function should be called on a dataplane core like the
> >    Rx function. They should not be called concurrently on the same
> >    queue.  
> 
> The first sentence looks fine. "They" (functions?, dataplane cores?) is 
> unclear for me in the second. May be the first one is simply sufficient.

Ok, I'll keep the first one at least, and see if I can reword the
second one to make it clear.

> >>> + *
> >>> + * @param port_id
> >>> + *  The port identifier of the Ethernet device.
> >>> + * @param queue_id
> >>> + *  The Rx queue identifier on this port.
> >>> + * @param offset
> >>> + *  The offset of the descriptor starting from tail (0 is the next
> >>> + *  packet to be received by the driver).
> >>> + * @return
> >>> + *  - (RTE_ETH_DESC_AVAIL): Descriptor is available for the hardware to
> >>> + *    receive a packet.
> >>> + *  - (RTE_ETH_DESC_DONE): Descriptor is done, it is filled by hw, but
> >>> + *    not yet processed by the driver (i.e. in the receive queue).
> >>> + *  - (RTE_ETH_DESC_USED): Descriptor is unavailable (hold by driver,
> >>> + *    not yet returned to hw).  
> >> It looks like it is the most suitable for descriptors which are reserved
> >> and never used.  
> > Can you give some more details about what is a reserved but never
> > used descriptor? (same question for Tx)  
> 
> Our HW has a requirement to keep few descriptors always unused (i.e. 
> some gap between tail and head). It is just a few descriptors, but 
> invalid descriptor status may misguide application. E.g. if Rx queue 
> size is 512 and offset 510, it will always be unused (since it is 
> reserved). It is not an indication that core is too slow and can't keep 
> the pace.

Understood.

I can change _USED into _UNAVAIL (add it for Tx), with the following
description:

- (RTE_ETH_DESC_UNAVAIL): Descriptor is unavailable: either hold by driver
  and not yet returned to hw, or reserved by the hardware.