RE: [PATCH 2/3] net/ionic: remove duplicate barriers

[Wathsala Wathawana Vithanage]

> -----Original Message-----
> From: Wathsala Wathawana Vithanage <wathsala.vithan...@arm.com>
> Sent: Monday, February 19, 2024 4:17 PM
> To: Andrew Boyer <andrew.bo...@amd.com>; dev@dpdk.org
> Cc: Neel Patel <neel.pa...@amd.com>; nd <n...@arm.com>; Honnappa
> Nagarahalli <honnappa.nagaraha...@arm.com>; nd <n...@arm.com>
> Subject: RE: [PATCH 2/3] net/ionic: remove duplicate barriers
> 
> Hi Andrew,
> 
> I think that this barrier may have been added to ensure any writes to q-
> >hw_index and q->head_idx complete before ionic_q_flush computes val.
> Dependency chains can also prevent reordering if that's the case this barrier 
> is
> not required.
> However, I have the following concern.
> 
> > diff --git a/drivers/net/ionic/ionic_main.c
> > b/drivers/net/ionic/ionic_main.c index 1f24f64a33..814bb3b8f4 100644
> > --- a/drivers/net/ionic/ionic_main.c
> > +++ b/drivers/net/ionic/ionic_main.c
> > @@ -223,7 +223,6 @@ ionic_adminq_post(struct ionic_lif *lif, struct
> > ionic_admin_ctx *ctx)
> >     q->head_idx = Q_NEXT_TO_POST(q, 1);
> >
> >     /* Ring doorbell */
> > -   rte_wmb();
> >     ionic_q_flush(q);
> >
> >  err_out:
> 
> Ionic_q_flush(q) uses q->hw_index and q->head_idx to compute the value of
> val which it writes to the address stored in q->db. I can see that 
> q->head_idx is
> updated before the removed rte_wmb(), therefore it's safe to assume that " q-
> >head_idx = Q_NEXT_TO_POST(q, 1)" and "val = IONIC_DBELL_QID(q-
> >hw_index) | q->head_idx" will maintain the program order due to that
> dependency. But I don't know if there exists a dependency chain over q-
> >hw_index. If not, then that may have been the motive behind this barrier.
> 

Since q->hw_index is also updated in the same CPU ionic_q_flush will
always see the correct value, consequently val should be always correct. 
It's safe to remove this barrier.

> > diff --git a/drivers/net/ionic/ionic_rxtx_sg.c
> > b/drivers/net/ionic/ionic_rxtx_sg.c
> > index e8dec99c04..3820fd850f 100644
> > --- a/drivers/net/ionic/ionic_rxtx_sg.c
> > +++ b/drivers/net/ionic/ionic_rxtx_sg.c
> > @@ -218,7 +218,6 @@ ionic_xmit_pkts_sg(void *tx_queue, struct
> rte_mbuf
> > **tx_pkts,
> >     }
> >
> >     if (nb_tx > 0) {
> > -           rte_wmb();
> >             ionic_txq_flush(q);
> >
> >             txq->last_wdog_cycles = rte_get_timer_cycles(); diff --git
> > a/drivers/net/ionic/ionic_rxtx_simple.c
> > b/drivers/net/ionic/ionic_rxtx_simple.c
> > index 9674b4d1df..4c9b9415ad 100644
> > --- a/drivers/net/ionic/ionic_rxtx_simple.c
> > +++ b/drivers/net/ionic/ionic_rxtx_simple.c
> > @@ -191,7 +191,6 @@ ionic_xmit_pkts(void *tx_queue, struct rte_mbuf
> > **tx_pkts,
> >     }
> >
> >     if (nb_tx > 0) {
> > -           rte_wmb();
> >             ionic_txq_flush(q);
> >
> >             txq->last_wdog_cycles = rte_get_timer_cycles();
> > --
> > 2.17.1