Subject: Re: [PATCH 0/6] In-kernel QMI handling

From: Bjorn Andersson <[email-masked]>
Date: Mon, 7 Aug 2017 21:45:07 -0700

On Mon 07 Aug 12:19 PDT 2017, Marcel Holtmann wrote:

> Hi Bjorn,
> 
> >>> This series starts by moving the common definitions of the QMUX
> >>> protocol to the
> >>> uapi header, as they are shared with clients - both in kernel and
> >>> userspace.
> >>> 
> >>> This series then introduces in-kernel helper functions for aiding the
> >>> handling
> >>> of QMI encoded messages in the kernel. QMI encoding is a wire-format
> >>> used in
> >>> exchanging messages between the majority of QRTR clients and
> >>> services.
> >> 
> >> This raises a few red-flags for me.
> > 
> > I'm glad it does. In discussions with the responsible team within
> > Qualcomm I've highlighted a number of concerns about enabling this
> > support in the kernel. Together we're continuously looking into what
> > should be pushed out to user space, and trying to not introduce
> > unnecessary new users.
> > 
> >> So far, we've kept almost everything QMI related in userspace and
> >> handled all QMI control-channel messages from libraries like libqmi or
> >> uqmi via the cdc-wdm driver and the "rmnet" interface via the qmi_wwan
> >> driver.  The kernel drivers just serve as the transport.
> >> 
> > 
> > The path that was taken to support the MSM-style devices was to
> > implement net/qrtr, which exposes a socket interface to abstract the
> > physical transports (QMUX or IPCROUTER in Qualcomm terminology).
> > 
> > As I share you view on letting the kernel handle the transportation only
> > the task of keeping track of registered services (service id -> node and
> > port mapping) was done in a user space process and so far we've only
> > ever have to deal with QMI encoded messages in various user space tools.
> 
> I think that the transport and multiplexing can be in the kernel as
> long as it is done as proper subsystem. Similar to Phonet or CAIF.
> Meaning it should have a well defined socket interface that can be
> easily used from userspace, but also a clean in-kernel interface
> handling.
> 

In a mobile Qualcomm device there's a few different components involved
here: message routing, QMUX protocol and QMI-encoding.

The downstream Qualcomm kernel implements the two first in the
IPCROUTER, upstream this is split between the kernel net/qrtr and a user
space service-register implementing the QMUX protocol for knowing where
services are located.

The common encoding of messages passed between endpoints of the message
routing is QMI, which is made an affair totally that of each client.

> If Qualcomm is supportive of this effort and is willing to actually
> assist and/or open some of the specs or interface descriptions, then
> this is a good thing. Service registration and cleanup is really done
> best in the kernel. Same applies to multiplexing. Trying to do
> multiplexing in userspace is always cumbersome and leads to overhead
> that is of no gain. For example within oFono, we had to force
> everything to go via oFono since it was the only sane way of handling
> it. Other approaches were error prone and full of race conditions. You
> need a central entity that can clean up.
> 

The current upstream solution depends on a collaboration between
net/qrtr and the user space service register for figuring out whom to
send messages to. After that muxing et al is handled by the socket
interface and service registry does not need to be involved.

Qualcomm is very supporting of this solution and we're collaborating on
transitioning "downstream" to use this implementation.

> For the definition of an UAPI to share some code, I am actually not
> sure that is such a good idea. For example the QMI code in oFono
> follows a way simpler approach. And I am not convinced that all the
> macros are actually beneficial. For example, the whole netlink macros
> are pretty cumbersome. Adding some Documentation/qmi.txt on how the
> wire format looks like and what is expected seems to be a way better
> approach.
> 

The socket interface provided by the kernel expects some knowledge of
the QMUX protocol, for service management. The majority of this
knowledge is already public, but I agree that it would be good to gather
this in a document. The common data structure for the control message is
what I've put in the uapi, as this is used by anyone dealing with
control messages.

When it comes to the QMI-encoded messages these are application
specific, just like e.g. protobuf definitions are application specific.

As the core infrastructure is becoming available upstream and boards
like the DB410c and DB820c aim to be supported by open solutions we will
have a natural place to discuss publication of at least some of the
application level protocols.

Regards,
Bjorn