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

[Konstantin Ananyev]

> -----Original Message-----
> From: Thomas Monjalon <thomas at monjalon.net>
> Sent: Friday, January 5, 2024 10:04 AM
> To: Jerin Jacob <jerinjacobk at gmail.com>
> Cc: Dumitrescu, Cristian <cristian.dumitrescu at intel.com>; Konstantin Ananyev <konstantin.ananyev at huawei.com>;
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> Subject: Re: [dpdk-dev] [RFC] ethdev: support Tx queue free descriptor query
> 
> 05/01/2024 10:57, 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
> 
> I'm fine with that.
> 

Yep, agree.
If we ever need  rte_eth_rx_queue_free_count() and rte_eth_tx_queue_used_count(),
it could be done via slow-path as Jerin outlined above, no need to waste entries in fp_ops
for that.