netdev - Re: [PATCH 17/18] net: iosm: readme file

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Date:   Sun, 17 Jan 2021 18:26:54 +0100
From:   Bjørn Mork <bjorn@...k.no>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     M Chetan Kumar <m.chetan.kumar@...el.com>, netdev@...r.kernel.org,
        linux-wireless@...r.kernel.org, johannes@...solutions.net,
        krishna.c.sudi@...el.com
Subject: Re: [PATCH 17/18] net: iosm: readme file

Sorry about being much too late into this discussion.  I'm not having
the bandwidth to read netdev anymore, and just stumbled across this now.

Andrew Lunn <andrew@...n.ch> writes:

> So, this is what all the Ethernet nonsense is all about. You have a
> session ID you need to somehow represent to user space. And you
> decided to use VLANs. But to use VLANs, you need an Ethernet
> header. So you added a bogus Ethernet header.

Actually, the original reasoning was the other way around.

The bogus ethernet header was added because I had seen the 3G modem
vendors do that for a few years already, in the modem firmware.  And I
didn't think enough about it to realize that it was a really bad idea,
or even that it was something I could change.  Or should change.

I cannot blame the MBIM sesison to VLAN mapping idea on anyone else.  As
far as I can remember, that was just something that popped up in my head
while working on the cdc_mbim driver. But it came as a consequence of
already having the bogus ethernet header.  And I didn't really
understand that I could define a new wwan subsystem with new device
types. I thought I had to use whatever was there already.

I was young and stupid. Now I'm not that young anymore ;-)

Never ever imagined that this would be replicated in another driver,
though.  That doesn't really make much sense.  We have learned by now,
haven't we?  This subject has been discussed a few times in the past,
and Johannes summary is my understanding as well:
"I don't think anyone likes that"

The DSS mapping sucks even more that the IPS mapping, BTW.  I don't
think there are any real users?  Not that I know of, at least.  DSS is
much better implmeneted as some per-session character device, as
requested by numerous people for years.  Sorry for not listening. Looks
like it is too late now.

> Is any of this VLAN stuff required by MBIM?

No.  It's my fault and mine alone.

> I suggest you throw away the pretence this is an Ethernet device. It
> is not.

I completely agree.  I wish I had gone for simple raw-ip devices both in
the qmi_wwan and cdc_mbim.  But qmi_wwan got them later, so there is
already support for such things in wwan userspace.

Bjørn