Re: [PATCH v17 3/3] bus: mhi: Add userspace client interface driver

On Fri, 2020-12-11 at 08:44 +0100, Greg KH wrote:

On Thu, Dec 10, 2020 at 11:04:11PM -0800, Hemant Kumar wrote:

quoted

This MHI client driver allows userspace clients to transfer
raw data between MHI device and host using standard file
operations.
Driver instantiates UCI device object which is associated to device
file node. UCI device object instantiates UCI channel object when
device
file node is opened. UCI channel object is used to manage MHI
channels
by calling MHI core APIs for read and write operations. MHI
channels
are started as part of device open(). MHI channels remain in start
state until last release() is called on UCI device file node.
Device
file node is created with format

/dev/<mhi_device_name>

Currently it supports QMI channel. libqmi is userspace MHI client
which
communicates to a QMI service using QMI channel. libqmi is a glib-
based
library for talking to WWAN modems and devices which speaks QMI
protocol.
For more information about libqmi please refer
https://www.freedesktop.org/wiki/Software/libqmi/

This says _what_ this is doing, but not _why_.

Why do you want to circumvent the normal user/kernel apis for this
type
of device and move the normal network handling logic out to
userspace?
What does that help with?  What does the current in-kernel api lack
that
this userspace interface is going to solve, and why can't the in-
kernel
api solve it instead?

You are pushing a common user/kernel api out of the kernel here, to
become very device-specific, with no apparent justification as to why
this is happening.

Also, because you are going around the existing network api, I will
need
the networking maintainers to ack this type of patch.

Just to re-iterate: QMI ~= AT commands ~= MBIM (not quite, but same
level)

We already do QMI-over-USB, or AT-over-CDC-ACM. This is QMI-over-MHI.

It's not networking data plane. It's WWAN device configuration.

There are no current kernel APIs for this, and I really don't think we
want there to be. The API surface is *huge* and we definitely don't
want that in-kernel.

Dan