Re: [EXT] Re: [net-next] net: dsa: felix: disable always guard band bit for TAS config

On Tue, Apr 20, 2021 at 10:28:45AM +0000, Xiaoliang Yang wrote:

Hi Vladimir,

On Tue, Apr 20, 2021 at 16:27:10AM +0800, Vladimir Oltean wrote:

quoted

On Tue, Apr 20, 2021 at 03:06:40AM +0000, Xiaoliang Yang wrote:

quoted

Hi Vladimir.

On Mon, Apr 19, 2021 at 20:38PM +0800, Vladimir Oltean wrote:

quoted

What is a scheduled queue? When time-aware scheduling is enabled on 
the port, why are some queues scheduled and some not?

The felix vsc9959 device can set SCH_TRAFFIC_QUEUES field bits to 
define which queue is scheduled. Only the set queues serves schedule 
traffic. In this driver we set all 8 queues to be scheduled in 
default, so all the traffic are schedule queues to schedule queue.

I understand this, what I don't really understand is the distinction
that the switch makes between 'scheduled' and 'non-scheduled'
traffic.  What else does this distinction affect, apart from the
guard bands added implicitly here? The tc-taprio qdisc has no notion
of 'scheduled' queues, all queues are 'scheduled'. Do we ever need
to set the scheduled queues mask to something other than 0xff? If
so, when and why?

Yes, it seems only affect the guard band. If disabling always guard
band bit, we can use SCH_TRAFFIC_QUEUES to determine which queue is
non-scheduled queue. Only the non-scheduled queue traffic will reserve
the guard band. But tc-taprio qdisc cannot set scheduled or
non-scheduled queue now. Adding this feature can be discussed in
future. 

It is not reasonable to add guardband in each queue traffic in
default, so I disable the always guard band bit for TAS config.

Ok, if true, then it makes sense to disable ALWAYS_GUARD_BAND_SCH_Q.