From: Arnd Bergmann <arnd@arndb.de>
To: Tom Gundersen <teg@jklm.no>
Subject: Re: [RFC v1 02/14] bus1: provide stub cdev /dev/bus1
Date: Sun, 30 Oct 2016 00:13:14 +0200
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Cc: Andy Lutomirski <luto@amacapital.net>,
        David Herrmann <dh.herrmann@gmail.com>,
        Hannes Reinecke <hare@suse.com>, Jiri Kosina <jikos@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Josh Triplett <josh@joshtriplett.org>, Greg KH <greg@kroah.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
References: <20161026191810.12275-1-dh.herrmann@gmail.com> <2469565.N6oiQLH2B8@wuerfel> <CAG-2HqXSGac7PXuHfG0Osh1LN_recr6knPYbCcY12Ks5GBDeYw@mail.gmail.com>
In-Reply-To: <CAG-2HqXSGac7PXuHfG0Osh1LN_recr6knPYbCcY12Ks5GBDeYw@mail.gmail.com>
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On Thursday 27 October 2016, Tom Gundersen wrote:
> On Thu, Oct 27, 2016 at 11:11 AM, Arnd Bergmann <arnd@arndb.de> wrote:
> > On Thursday, October 27, 2016 1:54:05 AM CEST Tom Gundersen wrote:
> >> On Thu, Oct 27, 2016 at 1:19 AM, Andy Lutomirski <luto@amacapital.net> wrote:
> >> > This may have been covered elsewhere, but could this use syscalls instead?
> >>
> >> Yes, syscalls would work essentially the same. For now, we are using a
> >> cdev as it makes it a lot more convenient to develop and test as an
> >> out-of-tree module, but that could be changed easily before the final
> >> submission, if that's what we want.
> >
> >
> > Generally speaking, I think syscalls would be appropriate here, and put
> > bus1 into a similar category as the other ipc interfaces (shm, msg, sem,
> > mqueue, ...).
> 
> Could you elaborate on why you think syscalls would be more
> appropriate than ioctls?

Linus already answered this, but I'd also add that core kernel
features just make sense to be syscalls, rather than stuffing
them in a random device driver.

> > - Have a mountable file system, and use open() on that to create
> >   connections. Advantages are that it's fairly easy to have one
> >   instance per fs-namespace, and you can have user-defined naming
> >   of objects in the file system.
> 
> Note that currently we only have one object (/dev/bus1) and each fd is
> disconnected from anything else on creation, so not sure what benefits
> a filesystem (or several instances of it) would give?

I have not tried to understand some of the main concepts of bus1,
so I simply assumed that there was some way of looking up handles
of other instances. Using a file system gives you a natural way
to look up resources by name the way we do e.g. for mq_open(),
and it lets you easy decide whether containers should share
a view of the same namespace by mounting the same instance of
the file system into them or having separate instances.

If you don't ever need to look up a handle by name in bus1, using
a mountable file system would not help you.

	Arnd