netdev - Re: [PATCH net-next v2 4/6] taprio: Add support for txtime-assist mode.

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Date:   Fri, 7 Jun 2019 15:02:43 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     "Patel, Vedang" <vedang.patel@...el.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
        "Gomes, Vinicius" <vinicius.gomes@...el.com>,
        "l@...ileo.org" <l@...ileo.org>,
        Murali Karicheri <m-karicheri2@...com>
Subject: Re: [PATCH net-next v2 4/6] taprio: Add support for txtime-assist
 mode.

On Fri, 7 Jun 2019 20:42:55 +0000, Patel, Vedang wrote:
> > Thanks for the changes, since you now validate no unknown flags are
> > passed, perhaps there is no need to check if flags are == ~0?
> > 
> > IS_ENABLED() could just do: (flags) & TCA_TAPRIO_ATTR_FLAG_TXTIME_ASSIST
> > No?
> >   
> This is specifically done so that user does not have to specify the
> offload flags when trying to install the another schedule which will
> be switched to at a later point of time (i.e. the admin schedule
> introduced in Vinicius’ last series). Setting taprio_flags to ~0
> will help us distinguish between the flags parameter not specified
> and flags set to 0.

I'm not super clear on this, because of backward compat you have to
treat attr not present as unset.  Let's see:

new qdisc:
 - flags attr = 0 -> txtime not used
 - flags attr = 1 -> txtime used
 -> no flags attr -> txtime not used
change qdisc:
 - flags attr = old flags attr -> leave unchanged
 - flags attr != old flags attr -> error
 - no flags attr -> leave txtime unchanged

Doesn't that cover the cases?  Were you planning to have no flag attr
on change mean disabled rather than no change?