netdev - Re: [PATCH 1/2] net: sch_generic: add flag IFF_FIFO_QUEUE to use pfifo

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Date:   Wed, 27 Mar 2019 13:53:40 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Marc Kleine-Budde <mkl@...gutronix.de>
Cc:     Jiri Pirko <jiri@...nulli.us>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Dave Taht <dave.taht@...il.com>,
        Jamal Hadi Salim <jhs@...atatu.com>, kernel@...gutronix.de,
        linux-can@...r.kernel.org, David Miller <davem@...emloft.net>
Subject: Re: [PATCH 1/2] net: sch_generic: add flag IFF_FIFO_QUEUE to use
 pfifo_fast as default scheduler

On Wed, Mar 27, 2019 at 1:11 PM Marc Kleine-Budde <mkl@...gutronix.de> wrote:
> Doubts aside, how should an arbitrary default qdisc per netdevice look
> like? Add a string "default_qdisc" to the netdev? Lookup qdisc by string
> during DOWN->UP transition? What do if that qdisc is not compiled into
> the kernel? Or rather use an array of qdiscs with one of sch_generic
> defaults?

I think you can just save a Qdisc_ops pointer in netdevice,
like how we install the default qdisc:

  38 const struct Qdisc_ops *default_qdisc_ops = &pfifo_fast_ops;
  39 EXPORT_SYMBOL(default_qdisc_ops);

And hard-code whatever default into your netdevice init code.

At least for pfifo_fast, you don't need to worry about module
loading. If you really do, you can call qdisc_lookup_default()
and request_module() like what qdisc_set_default() does.
Of course, you can refactor qdisc_set_default() and call it too.

Thanks.