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Date:   Tue, 18 Apr 2023 13:25:38 +0200
From:   Guillaume Nault <gnault@redhat.com>
To:     Samuel Thibault <samuel.thibault@ens-lyon.org>,
        James Chapman <jchapman@katalix.com>, tparkin@katalix.com,
        edumazet@google.com, davem@davemloft.net, kuba@kernel.org,
        pabeni@redhat.com, corbet@lwn.net, netdev@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] PPPoL2TP: Add more code snippets
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On Tue, Apr 18, 2023 at 12:31:40PM +0200, Samuel Thibault wrote:
> Guillaume Nault, le mar. 18 avril 2023 12:17:55 +0200, a ecrit:
> > On Tue, Apr 18, 2023 at 11:11:48AM +0200, Samuel Thibault wrote:
> > > Guillaume Nault, le mar. 18 avril 2023 11:06:51 +0200, a ecrit:
> > > > On Tue, Apr 18, 2023 at 10:53:23AM +0200, Samuel Thibault wrote:
> > > > > Guillaume Nault, le mar. 18 avril 2023 10:34:03 +0200, a ecrit:
> > > > > > PPPIOCBRIDGECHAN's description
> > > > > > belongs to Documentation/networking/ppp_generic.rst, where it's already
> > > > > > documented.
> > > > > 
> > > > > Yes but that's hard to find out when you're looking from the L2TP end.
> > > > 
> > > > That's why I proposed linking to ppp_generic.rst.
> > > 
> > > Yes, but it's still not obvious to L2TP people that it's a ppp channel
> > > that you have to bridge. Really, having that 20-line snippet available
> > > would have saved me some head-scratching time.
> > 
> > But the reverse is also true: someone looking at the PPP documentation
> > is probably not going to realise that PPP sample code have been put in
> > the L2TP doc.
> 
> Yes, but for PPP people it is obvious that you'll want to bridge two
> channels.
> 
> The point of the code is not really the bridging ioctl call, but the
> fact that you have to use PPPIOCGCHAN over the two sessions, then open
> a ppp channel, to be able to make the bridging ioctl call. *That*
> is what is really not obvious, and will not actually fit in the PPP
> documentation. Of course we could move the few ppp-only lines to the PPP
> documentation, but I really don't see the point: that part is obvious in
> the PPP context.

For PPPIOCGCHAN, I agree it should be documented in l2tp.rst. This
ioctl is common to all PPPOX sockets, but it wouldn't make sense to
have a separate document just for it. And L2TP is the only PPPOX user
that is documented as far as I know.

As I said in my previous reply, a simple L2TP example that goes until PPP
channel and unit creation is fine. But any more advanced use of the PPP
API should be documented in the PPP documentation.

I mean, these files document the API of their corresponding modules,
their scope should be limitted to that (the PPP and L2TP layers are
really different).

That shouldn't preclude anyone from describing how to combine L2TP, PPP
and others to cover more advanced use cases. It's just better done in a
different file.

> Samuel
>