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

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Date:   Tue, 20 Apr 2021 10:28:45 +0000
From:   Xiaoliang Yang <xiaoliang.yang_1@....com>
To:     Vladimir Oltean <olteanv@...il.com>
CC:     "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
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        "allan.nielsen@...rochip.com" <allan.nielsen@...rochip.com>,
        "joergen.andreasen@...rochip.com" <joergen.andreasen@...rochip.com>,
        "colin.king@...onical.com" <colin.king@...onical.com>,
        Po Liu <po.liu@....com>, Mingkai Hu <mingkai.hu@....com>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Vladimir Oltean <vladimir.oltean@....com>,
        Leo Li <leoyang.li@....com>
Subject: RE: [EXT] Re: [net-next] net: dsa: felix: disable always guard band
 bit for TAS config

Hi Vladimir,

On Tue, Apr 20, 2021 at 16:27:10AM +0800, Vladimir Oltean wrote:
>
> On Tue, Apr 20, 2021 at 03:06:40AM +0000, Xiaoliang Yang wrote:
>> Hi Vladimir.
>>
>> On Mon, Apr 19, 2021 at 20:38PM +0800, Vladimir Oltean wrote:
>> >
>> >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.

Thanks,
Xiaoliang Yang