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From: Guillaume Nault <gnault@redhat.com>
To: "Pali Rohár" <pali@kernel.org>
Cc: Jakub Kicinski <kuba@kernel.org>,
	Paul Mackerras <paulus@samba.org>,
	"David S. Miller" <davem@davemloft.net>,
	linux-ppp@vger.kernel.org, netdev@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] ppp: Add rtnl attribute IFLA_PPP_UNIT_ID for specifying ppp unit id
Date: Wed, 11 Aug 2021 19:19:18 +0200	[thread overview]
Message-ID: <20210811171918.GD15488@pc-32.home> (raw)
In-Reply-To: <20210810160450.eluiktsp7oentxo3@pali>

On Tue, Aug 10, 2021 at 06:04:50PM +0200, Pali Rohár wrote:
> On Tuesday 10 August 2021 17:39:41 Guillaume Nault wrote:
> > On Mon, Aug 09, 2021 at 09:31:09PM +0200, Pali Rohár wrote:
> > > Better to wait. I would like hear some comments / review on this patch
> > > if this is the correct approach as it adds a new API/ABI for userspace.
> > 
> > Personally I don't understand the use case for setting the ppp unit at
> > creation time.
> 
> I know about two use cases:
> 
> * ppp unit id is used for generating network interface name. So if you
>   want interface name ppp10 then you request for unit id 10. It is
>   somehow common that when ppp interface has prefix "ppp" in its name
>   then it is followed by unit id. Seems that existing ppp applications
>   which use "ppp<num>" naming expects this. But of course you do not
>   have to use this convention and rename interfaces as you want.

Really, with the netlink API, the interface name has to be set with
IFLA_IFNAME. There's no point in adding a new attribute just to have a
side effect on the device name.

> * Some of ppp ioctls use unit id. So you may want to use some specific
>   number for some network interface. So e.g. unit id 1 will be always
>   for /dev/ttyUSB1.

But what's the point of forcing unit id 1 for a particular interface?
One can easily get the assigned unit id with ioctl(PPPIOCGUNIT).

> > I didn't implement it on purpose when creating the
> > netlink interface, as I didn't have any use case.
> > 
> > On the other hand, adding the ppp unit in the netlink dump is probably
> > useful.
> 
> Yes, this could be really useful as currently if you ask netlink to
> create a new ppp interface you have to use ioctl to retrieve this unit
> id. But ppp currently does not provide netlink dump operation.
> 
> Also it could be useful for this "bug":
> https://lore.kernel.org/netdev/20210807132703.26303-1-pali@kernel.org/t/#u

This patch itself makes sense, but how is that related to unit id?

> And with unit id there also another issue:
> https://lore.kernel.org/netdev/20210807160050.17687-1-pali@kernel.org/t/#u

This patch shows why linking unit id and interface name are a bad idea.

Instead of adding more complexity with unit id, I'd prefer to have a
new netlink attribute that says "don't generate the interface name
based on the unit id". That's how the original implementation worked by
the way and I'm really sad I accepted to change it...

> But due to how it is used we probably have to deal with it how ppp unit
> id are defined and assigned...
> 


  reply	other threads:[~2021-08-11 17:19 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-07 16:37 Pali Rohár
2021-08-09 19:25 ` Jakub Kicinski
2021-08-09 19:31   ` Pali Rohár
2021-08-10 15:39     ` Guillaume Nault
2021-08-10 16:04       ` Pali Rohár
2021-08-11 17:19         ` Guillaume Nault [this message]
2021-08-11 17:54           ` Pali Rohár
2021-08-12  9:19             ` Guillaume Nault
2021-08-12 14:09               ` Pali Rohár
2021-08-12 19:12                 ` Guillaume Nault
     [not found]       ` <BN0P223MB0327A247724B7AE211D2E84EA7F79@BN0P223MB0327.NAMP223.PROD.OUTLOOK.COM>
2021-08-10 17:16         ` Pali Rohár
2021-08-10 18:11           ` James Carlson
2021-08-11 17:38             ` Guillaume Nault
2021-08-11 18:04               ` Pali Rohár
2021-08-12  9:28                 ` Guillaume Nault
2021-08-12 13:48                   ` Pali Rohár
2021-08-12 18:26                     ` Guillaume Nault
2021-08-12 19:04                       ` Pali Rohár
2021-08-16 16:11                         ` Guillaume Nault
2021-08-16 16:23                           ` Pali Rohár
2021-08-17 16:05                             ` Guillaume Nault
2021-08-17 16:21                               ` Pali Rohár
2022-07-09 12:09                                 ` Pali Rohár
2022-07-12 17:34                                   ` Guillaume Nault

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