net: wwan: Add Qualcomm BAM-DMUX WWAN network driver

net: wwan: Add Qualcomm BAM-DMUX WWAN network driver

The BAM Data Multiplexer provides access to the network data channels of
modems integrated into many older Qualcomm SoCs, e.g. Qualcomm MSM8916 or
MSM8974. It is built using a simple protocol layer on top of a DMA engine
(Qualcomm BAM) and bidirectional interrupts to coordinate power control.

The modem announces a fixed set of channels by sending an OPEN command.
The driver exports each channel as separate network interface so that
a connection can be established via QMI from userspace. The network
interface can work either in Ethernet or Raw-IP mode (configurable via
QMI). However, Ethernet mode seems to be broken with most firmwares
(network packets are actually received as Raw-IP), therefore the driver
only supports Raw-IP mode.

Note that the control channel (QMI/AT) is entirely separate from
BAM-DMUX and is already supported by the RPMSG_WWAN_CTRL driver.

The driver uses runtime PM to coordinate power control with the modem.
TX/RX buffers are put in a kind of "ring queue" and submitted via
the bam_dma driver of the DMAEngine subsystem.

The basic architecture looks roughly like this:

                   +------------+                +-------+
         [IPv4/6]  |  BAM-DMUX  |                |       |
         [Data...] |            |                |       |
        ---------->|wwan0       | [DMUX chan: x] |       |
         [IPv4/6]  | (chan: 0)  | [IPv4/6]       |       |
         [Data...] |            | [Data...]      |       |
        ---------->|wwan1       |--------------->| Modem |
                   | (chan: 1)  |      BAM       |       |
         [IPv4/6]  | ...        |  (DMA Engine)  |       |
         [Data...] |            |                |       |
        ---------->|wwan7       |                |       |
                   | (chan: 7)  |                |       |
                   +------------+                +-------+

Note that some newer firmware versions support QMAP ("rmnet" driver)
as additional multiplexing layer on top of BAM-DMUX, but this is not
currently supported by this driver.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: David S. Miller <davem@davemloft.net>