[dpdk-dev] [PATCH 1/6] ethdev: add descriptor status API
Olivier Matz
olivier.matz at 6wind.com
Thu Mar 2 15:54:32 CET 2017
> >>> + * 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.
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