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

[Wathsala Wathawana Vithanage]

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.

> 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