Re: [PATCH net-next v3 2/8] taprio: Add support for frame preemption offload

Vinicius Costa Gomes
2021-01-29T21:14:33Z

Vladimir Oltean <vladimir.oltean@nxp.com> writes:

> On Fri, Jan 22, 2021 at 02:44:47PM -0800, Vinicius Costa Gomes wrote:
>> + /* It's valid to enable frame preemption without any kind of
>> + * offloading being enabled, so keep it separated.
>> + */
>> + if (tb[TCA_TAPRIO_ATTR_PREEMPT_TCS]) {
>> + u32 preempt = nla_get_u32(tb[TCA_TAPRIO_ATTR_PREEMPT_TCS]);
>> + struct tc_preempt_qopt_offload qopt = { };
>> +
>> + if (preempt == U32_MAX) {
>> + NL_SET_ERR_MSG(extack, "At least one queue must be not be preemptible");
>> + err = -EINVAL;
>> + goto free_sched;
>> + }
>> +
>> + qopt.preemptible_queues = tc_map_to_queue_mask(dev, preempt);
>> +
>> + err = dev->netdev_ops->ndo_setup_tc(dev, TC_SETUP_PREEMPT,
>> + &qopt);
>> + if (err)
>> + goto free_sched;
>> +
>> + q->preemptible_tcs = preempt;
>> + }
>> +
>
> First I'm interested in the means: why check for preempt == U32_MAX when
> you determine that all traffic classes are preemptible? What if less
> than 32 traffic classes are used by the netdev? The check will be
> bypassed, won't it?

Good catch :-)

I wanted to have this (at least one express queue) handled in a
centralized way, but perhaps this should be handled best per driver.

>
> Secondly, why should at least one queue be preemptible? What's wrong
> with frame preemption being triggered by a tc-taprio window smaller than
> the packet size? This can happen regardless of traffic class.

It's the opposite, at least one queue needs to be marked
express/non-preemptible. But as I said above, perhaps this should be
handled in a per-driver way. I will remove this from taprio.

I think removing this check/limitation from taprio should solve the
second part of your question, right?

Cheers,
--
Vinicius