Subject: Re: [PATCH net-next] igc: Lift TAPRIO schedule restriction

From: Vinicius Costa Gomes <[email-masked]>
Date: Mon, 06 Jun 2022 16:55:21 -0700

Hi Kurt,

Kurt Kanzenbach <[email-masked]> writes:

> Add support for Qbv schedules where one queue stays open
> in consecutive entries. Currently that's not supported.
>
> Example schedule:
>
> |tc qdisc replace dev ${INTERFACE} handle 100 parent root taprio num_tc 3 \
> |   map 2 2 1 0 2 2 2 2 2 2 2 2 2 2 2 2 \
> |   queues 1@0 1@1 2@2 \
> |   base-time ${BASETIME} \
> |   sched-entry S 0x01 300000 \ # Stream High/Low
> |   sched-entry S 0x06 500000 \ # Management and Best Effort
> |   sched-entry S 0x04 200000 \ # Best Effort
> |   flags 0x02
>
> Signed-off-by: Kurt Kanzenbach <[email-masked]>
> ---
>  drivers/net/ethernet/intel/igc/igc_main.c | 23 +++++++++++++++++------
>  1 file changed, 17 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/net/ethernet/intel/igc/igc_main.c b/drivers/net/ethernet/intel/igc/igc_main.c
> index ae17af44fe02..4758bdbe5df3 100644
> --- a/drivers/net/ethernet/intel/igc/igc_main.c
> +++ b/drivers/net/ethernet/intel/igc/igc_main.c
> @@ -5813,9 +5813,10 @@ static bool validate_schedule(struct igc_adapter *adapter,
>  		return false;
>  
>  	for (n = 0; n < qopt->num_entries; n++) {
> -		const struct tc_taprio_sched_entry *e;
> +		const struct tc_taprio_sched_entry *e, *prev;
>  		int i;
>  
> +		prev = n ? &qopt->entries[n - 1] : NULL;
>  		e = &qopt->entries[n];
>  
>  		/* i225 only supports "global" frame preemption
> @@ -5828,7 +5829,12 @@ static bool validate_schedule(struct igc_adapter *adapter,
>  			if (e->gate_mask & BIT(i))
>  				queue_uses[i]++;
>  
> -			if (queue_uses[i] > 1)
> +			/* There are limitations: A single queue cannot be
> +			 * opened and closed multiple times per cycle unless the
> +			 * gate stays open. Check for it.
> +			 */
> +			if (queue_uses[i] > 1 &&
> +			    !(prev->gate_mask & BIT(i)))

Perhaps I am missing something, I didn't try to run, but not checking if
'prev' can be NULL, could lead to crashes for some schedules, no?

What I have in mind is a schedule that queue 0 is mentioned multiple
times, for example:

 |   sched-entry S 0x01 300000 \ # Stream High/Low
 |   sched-entry S 0x03 500000 \ # Management and Best Effort
 |   sched-entry S 0x05 200000 \ # Best Effort

Anyway, looks much cleaner than what I had in mind when I wrote that
fixme.

Cheers,
-- 
Vinicius