netdev - Re: [RFC PATCH net-next 4/4] net: wwan: Add Qualcomm BAM-DMUX WWAN network driver

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Date:   Mon, 19 Jul 2021 18:01:33 +0200
From:   Loic Poulain <loic.poulain@...aro.org>
To:     Stephan Gerhold <stephan@...hold.net>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Sergey Ryazanov <ryazanov.s.a@...il.com>,
        Johannes Berg <johannes@...solutions.net>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Andy Gross <agross@...nel.org>, Vinod Koul <vkoul@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Aleksander Morgado <aleksander@...ksander.es>,
        Network Development <netdev@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        dmaengine@...r.kernel.org, devicetree <devicetree@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        phone-devel@...r.kernel.org, ~postmarketos/upstreaming@...ts.sr.ht,
        Jeffrey Hugo <jeffrey.l.hugo@...il.com>
Subject: Re: [RFC PATCH net-next 4/4] net: wwan: Add Qualcomm BAM-DMUX WWAN
 network driver

Hi Stephan,


On Mon, 19 Jul 2021 at 17:01, Stephan Gerhold <stephan@...hold.net> wrote:
>
> I'm not sure how to integrate the driver with the WWAN subsystem yet.
> At the moment the driver creates network interfaces for all channels
> announced by the modem, it does not make use of the WWAN link management
> yet. Unfortunately, this is a bit complicated:
>
> Both QMAP and the built-in multiplexing layer might be needed at some point.
> There are firmware versions that do not support QMAP and the other way around
> (the built-in multiplexing was disabled on very recent firmware versions).
> Only userspace can check if QMAP is supported in the firmware (via QMI).
>
> I could ignore QMAP completely for now but I think someone will show up
> who will need this eventually. And if there is going to be common code for
> QMAP/rmnet link management it would be nice if BAM-DMUX could also make
> use of it.

I have this on my TODO list for mhi-net QMAP.

> But the question is, how could this look like? How do we know if we should
> create a link for QMAP or a BAM-DMUX channel? Does it even make sense
> to manage the 1-8 channels via the WWAN link management?

Couldn't it be specified via dts (property or different compatible
string)? would it make sense to have two drivers (with common core) to
manage either the multi-bam channel or newer QMAP based single
bam-channel modems.

>
> Another problem is that the WWAN subsystem currently creates all network
> interfaces below the common WWAN device. This means that userspace like
> ModemManager has no way to check which driver provides them. This is
> necessary though to decide how to set it up via QMI (ModemManager uses it).

Well, I have quite a similar concern since I'm currently porting
mhi-net mbim to wwan framework, and I was thinking about not making
wwan device parent of the network link/netdev (in the same way as
wlan0 is not child of ieee80211 device), but not sure if it's a good
idea or not since we can not really consider driver name part of the
uapi.

The way links are created is normally abstracted, so if you know which
bam variant you have from wwan network driver side (e.g. via dts), you
should have nothing to check on the user side, except the session id.

Regards,
Loic