netdev - Re: [RFC PATCH net-next 1/3] net: rtnetlink: Add link-down reason to RTNL messages

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Message-ID: <20190319084206.6553a9f1@shemminger-XPS-13-9360>
Date:   Tue, 19 Mar 2019 08:42:06 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Petr Machata <petrm@...lanox.com>
Cc:     Andrew Lunn <andrew@...n.ch>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Jiri Pirko <jiri@...lanox.com>,
        Ido Schimmel <idosch@...lanox.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        Tariq Toukan <tariqt@...lanox.com>,
        "jakub.kicinski@...ronome.com" <jakub.kicinski@...ronome.com>
Subject: Re: [RFC PATCH net-next 1/3] net: rtnetlink: Add link-down reason
 to RTNL messages

On Tue, 19 Mar 2019 10:18:00 +0000
Petr Machata <petrm@...lanox.com> wrote:

> Stephen Hemminger <stephen@...workplumber.org> writes:
> 
> > On Mon, 18 Mar 2019 15:02:53 +0100
> > Andrew Lunn <andrew@...n.ch> wrote:
> >  
> >> On Mon, Mar 18, 2019 at 01:15:41PM +0000, Petr Machata wrote:  
> >> >
> >> > Andrew Lunn <andrew@...n.ch> writes:
> >> >  
> >> > >> +enum rtnl_link_down_reason_major {
> >> > >> +	RTNL_LDR_OTHER,  
> >> > >
> >> > > Does 'other' make any sense? Seem better to just not report anything
> >> > > at all, or add a comment that more reasons should be added at the end
> >> > > to reflect whatever the hardware or software can determine.  
> >> >
> >> > You still have the minor code to give you some information.  
> >>
> >> The problem i have with OTHER, is that you know it is not NO_CABLE,
> >> UNSUPPORTED_CABLE, AUTONEG_FAILURE, etc. But for people to know what
> >> OTHER cannot be, they have to know all the codes.
> >>
> >> But then later, some other driver writer does the right thing, adds a
> >> new value to the end for a code they can detect. Say for example
> >> SFP_OVERHEATED.  This happened to be what the previous driver was
> >> using for OTHER. Now we have one driver returning SFP_OVERHEATED and
> >> the older driver OTHER. So OTHER no longer actually mean 'other', it
> >> just means something random, which could actually be the same as one
> >> of the listed codes.
> >>
> >> You can stop this from happening by not having OTHER. Always add a new
> >> code if there is something you can report, but there currently is no
> >> code for it. And the userspace tool should just print the decimal
> >> value if it does not know what text to translate it into.  
> >
> > Gut feel is that enumerated values are going to grow and grow and be
> > long term API headache.
> >
> > Would it be possible to use a string like the external ack error
> > message?  
> 
> It would, but then if any automated tools want to make use of it beyond
> just blindly displaying it, they will need to parse it with all the
> usual problems. In the end the string itself becomes the API anyway.
> 
> Adding a string would make sense as an extra piece of information, not
> as the primary channel. Extack is like this as well, the primary channel
> there is errno.

The problem with codes is that without some standard (like IETF) the
values are very system specific and likely to get lots of of version churn.