RE: [dpdk-dev] [v1] ethdev: support Tx queue used count

[Konstantin Ananyev <konstantin.ananyev@huawei.com>]

Hi Jerin,

> > > Introduce a new API to retrieve the number of used 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@marvell.com>
> 
> > > @@ -6803,6 +6803,80 @@ rte_eth_recycle_mbufs(uint16_t rx_port_id, uint16_t rx_queue_id,
> > >  __rte_experimental
> > >  int rte_eth_buffer_split_get_supported_hdr_ptypes(uint16_t port_id, uint32_t *ptypes, int num);
> > >
> > > +/**
> > > + * @warning
> > > + * @b EXPERIMENTAL: this API may change, or be removed, without prior notice
> > > + *
> > > + * Get the number of used descriptors of a Tx queue
> > > + *
> > > + * This function retrieves the number of used descriptors of a transmit queue.
> > > + * Applications can use this API in the fast path to inspect Tx queue occupancy and take
> > > + * appropriate actions based on the available free descriptors.
> > > + * An example action could be implementing the Random Early Discard (RED).
> >
> > Sorry, I probably misunderstood your previous mails, but wouldn't it be more convenient
> > for user to have rte_eth_tx_queue_free_count(...) as fast-op, and
> > have rte_eth_tx_queue_count(...) {  queue_txd_num - rte_eth_tx_queue_free_count(...);}
> > as a slow-path function in rte_ethdev.c?
> 
> The general feedback is to align with the Rx queue API, specifically
> rte_eth_rx_queue_count,
> and it's noted that there is no equivalent rte_eth_rx_queue_free_count.
> 
> Given that the free count can be obtained by subtracting the used
> count from queue_txd_num,
> it is considered that either approach is acceptable.
> 
> The application configures queue_txd_num with tx_queue_setup(), and
> the application can store that value in its structure.
> This would enable fast-path usage for both base cases (whether the
> application needs information about free or used descriptors)
> with just one API(rte_eth_tx_queue_count())

Right now I don't use these functions, but if I think what most people are interested in:
- how many packets you can receive immediately (rx_queue_count)
- how many packets you can transmit immediately (tx_queue_free_count)
Sure, I understand that user can store txd_num  somewhere and then do subtraction himself. 
Though it means more effort for the user, and the only reason for that, as I can see,
is to have RX and TX function naming symmetric.
Which seems much less improtant to me comparing to user convenience.
Anyway, as I stated above, I don't use these functions right now,
so if the majority of users are happy with current approach, I would not insist :)  
Konstantin