RE: [PATCH net] net: mana: Fix doorbell out of order violation and avoid unnecessary doorbell rings

[Dexuan Cui]

> From: longli@xxxxxxxxxxxxxxxxx <longli@xxxxxxxxxxxxxxxxx>
> Sent: Monday, August 5, 2024 4:38 PM
> [...]
> After napi_complete_done() is called, another NAPI may be running on
> another CPU and ring the doorbell before the current CPU does. When

Can you please share more details about "another NAPI"? Is it about busy_poll?

> combined with unnecessary rings when there is no need to ARM the CQ, this
> triggers error paths in the hardware.
>
> Fix this by always ring the doorbell in sequence and avoid unnecessary
> rings.

I'm not sure what "error paths in the hardware" means. It's better to describe
the user-visible consequence.

Maybe this is clearer:

When there is no need to arm the CQ from NAPI's perspective, the driver must
not combine "too many" arming operations due to a MANA hardware requirement:
the driver must ring the doorbell at least once within every 8 wraparounds of the CQ,
otherwise "XXX" would happen. //Dexuan: I don't know what the "XXX" is

Add a per-CQ counter cq->work_done_since_doorbell, and make sure the CQ is
armed within 4 wraparounds of the CQ. //Dexuan: why not 8 or 7?

 
> +	if (w < cq->budget) {
> +		mana_gd_ring_cq(gdma_queue, SET_ARM_BIT);
> +		cq->work_done_since_doorbell = 0;
> +		napi_complete_done(&cq->napi, w);
> +	} else if (cq->work_done_since_doorbell >
> +		   cq->gdma_cq->queue_size / COMP_ENTRY_SIZE * 4) {
> +		/* MANA hardware requires at least one doorbell ring every 8
s/ring every 8/arming within every 8/ ?

> +		 * wraparounds of CQ even there is no need to ARM. This
> driver

s/ARM/arming/ ?
s/even/even if/ ?

Thanks,
Dexuan