[dpdk-dev] [RFC] ethdev: support Tx queue free descriptor query

[Jerin Jacob]

On Thu, Jan 4, 2024 at 11:59 PM Thomas Monjalon <thomas at monjalon.net> wrote:
>
> 04/01/2024 15:21, Konstantin Ananyev:
> >
> > > > > Introduce a new API to retrieve the number of available free descriptors
> > > > > in a Tx queue. Applications can leverage this API in the fast path to
> > > > > inspect the Tx queue occupancy and take appropriate actions based on the
> > > > > available free descriptors.
> > > > >
> > > > > A notable use case could be implementing Random Early Discard (RED)
> > > > > in software based on Tx queue occupancy.
> > > > >
> > > > > Signed-off-by: Jerin Jacob <jerinj at marvell.com>
> > > >
> > > > I think having an API to get the number of free descriptors per queue is a good idea. Why have it only for TX queues and not for RX
> > > queues as well?
> > >
> > > I see no harm in adding for Rx as well. I think, it is better to have
> > > separate API for each instead of adding argument as it is fast path
> > > API.
> > > If so, we could add a new API when there is any PMD implementation or
> > > need for this.
> >
> > I think for RX we already have similar one:
> > /** @internal Get number of used descriptors on a receive queue. */
> > typedef uint32_t (*eth_rx_queue_count_t)(void *rxq);
>
> rte_eth_rx_queue_count() gives the number of Rx used descriptors
> rte_eth_rx_descriptor_status() gives the status of one Rx descriptor
> rte_eth_tx_descriptor_status() gives the status of one Tx descriptor
>
> This patch is adding a function to get Tx available descriptors,
> rte_eth_tx_queue_free_desc_get().
> I can see a symmetry with rte_eth_rx_queue_count().
> For consistency I would rename it to rte_eth_tx_queue_free_count().
>
> Should we add rte_eth_tx_queue_count() and rte_eth_rx_queue_free_count()?

IMO, rte_eth_rx_queue_free_count() is enough as
used count =  total desc number(configured via nb_tx_desc with
rte_eth_tx_queue_setup())  - free count

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