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

From: Kurt Kanzenbach <[email-masked]>
Date: Tue, 07 Jun 2022 09:06:53 +0200

Hi Vinicius,

On Mon Jun 06 2022, Vinicius Costa Gomes wrote:
> 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?

My thinking was that `prev` can never be NULL, as `queue_uses[i] > 1` is
checked first. This condition can only be true if there are at least two
entries.

>
> 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
>

So, this schedule works with the proposed patch. Queue 0 is opened in
all three entries. My debug code shows:

|tc-6145    [010] .......   616.190589: igc_setup_tc: Qbv configuration:
|tc-6145    [010] .......   616.190592: igc_setup_tc: Queue 0 -- start_time=0 [ns]
|tc-6145    [010] .......   616.190592: igc_setup_tc: Queue 0 -- end_time=1000000 [ns]
|tc-6145    [010] .......   616.190593: igc_setup_tc: Queue 1 -- start_time=300000 [ns]
|tc-6145    [010] .......   616.190593: igc_setup_tc: Queue 1 -- end_time=800000 [ns]
|tc-6145    [010] .......   616.190593: igc_setup_tc: Queue 2 -- start_time=800000 [ns]
|tc-6145    [010] .......   616.190594: igc_setup_tc: Queue 2 -- end_time=1000000 [ns]
|tc-6145    [010] .......   616.190594: igc_setup_tc: Queue 3 -- start_time=800000 [ns]
|tc-6145    [010] .......   616.190594: igc_setup_tc: Queue 3 -- end_time=1000000 [ns]

Anyway, I'd appreciate some testing on your side too :).

Thanks,
Kurt