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

[Sergey Ryazanov <ryazanov.s.a@gmail.com>]

Hello Stephan,

On Wed, Jul 21, 2021 at 3:17 PM Stephan Gerhold <stephan@gerhold.net> wrote:
> On Tue, Jul 20, 2021 at 12:10:42PM +0300, Sergey Ryazanov wrote:
>> On Mon, Jul 19, 2021 at 6:01 PM Stephan Gerhold <stephan@gerhold.net> wrote:
>>> 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.
>>>
>>> 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...] |            |                |       |
>>>         ---------->|rmnet0      | [DMUX chan: x] |       |
>>>          [IPv4/6]  | (chan: 0)  | [IPv4/6]       |       |
>>>          [Data...] |            | [Data...]      |       |
>>>         ---------->|rmnet1      |--------------->| Modem |
>>>                    | (chan: 1)  |      BAM       |       |
>>>          [IPv4/6]  | ...        |  (DMA Engine)  |       |
>>>          [Data...] |            |                |       |
>>>         ---------->|rmnet7      |                |       |
>>>                    | (chan: 7)  |                |       |
>>>                    +------------+                +-------+
>>>
>>> However, on newer SoCs/firmware versions Qualcomm began gradually moving
>>> to QMAP (rmnet driver) as backend-independent protocol for multiplexing
>>> and data aggegration. Some firmware versions allow using QMAP on top of
>>> BAM-DMUX (effectively resulting in a second multiplexing layer plus data
>>> aggregation). The architecture with QMAP would look roughly like this:
>>>
>>>            +-------------+           +------------+                  +-------+
>>>  [IPv4/6]  |    RMNET    |           |  BAM-DMUX  |                  |       |
>>>  [Data...] |             |           |            | [DMUX chan: 0]   |       |
>>> ---------->|rmnet_data1  |     ----->|rmnet0      | [QMAP mux-id: x] |       |
>>>            | (mux-id: 1) |     |     | (chan: 0)  | [IPv4/6]         |       |
>>>            |             |     |     |            | [Data...]        |       |
>>>  [IPv4/6]  | ...         |------     |            |----------------->| Modem |
>>>  [Data...] |             |           |            |       BAM        |       |
>>> ---------->|rmnet_data42 | [QMAP: x] |[rmnet1]    |   (DMA Engine)   |       |
>>>            | (mux-id: 42)| [IPv4/6]  |... unused! |                  |       |
>>>            |             | [Data...] |[rmnet7]    |                  |       |
>>>            |             |           |            |                  |       |
>>>            +-------------+           +------------+                  +-------+
>>>
>>> In this case, rmnet1-7 would remain unused. The firmware used on the most
>>> recent SoCs with BAM-DMUX even seems to announce only a single BAM-DMUX
>>> channel (rmnet0), which makes QMAP the only option for multiplexing there.
>>>
>>> So far the driver is mainly tested on various smartphones/tablets based on
>>> Qualcomm MSM8916/MSM8974 without QMAP. It looks like QMAP depends on a MTU
>>> negotiation feature in BAM-DMUX which is not yet supported by the driver.
>>>
>>> Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
>>> ---
>>> Note that this is my first network driver, so I apologize in advance
>>> if I made some obvious mistakes. :)
>>>
>>> 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 am not very familiar with the Qualcomm protocols and am just curious
>> whether BAM-DMUX has any control (management) channels or only IPv4/v6
>> data channels?
>>
>> The WWAN subsystem began as a framework for exporting management
>> interfaces (MBIM, AT, etc.) to user space. And then the network
>> interfaces (data channels) management interface was added to
>> facilitate management of devices with multiple data channels. That is
>> why I am curious about the BAM-DMUX device management interface or in
>> other words, how a user space application could control the modem
>> work?
>
> Sorry for the confusion! It's briefly mentioned in the Kconfig option
> but I should have made this more clear in the commit message. It was so
> long already that I wasn't sure where to put it. :)
>
> BAM-DMUX does not have any control channels. Instead I use it together
> with the rpmsg_wwan_ctrl driver [1] that I already submitted for 5.14.
> The control/data channels are pretty much separate in this setup and
> don't have much to do with each other.
>
> I also had a short overview of some of the many different modem
> protocols Qualcomm has come up with in a related RFC for that driver,
> see [2] if you are curious.
>
> I hope that clarifies some things, please let me know if I should
> explain something better! :)
>
> [1]: https://lore.kernel.org/netdev/20210618173611.134685-3-stephan@gerhold.net/
> [2]: https://lore.kernel.org/netdev/YLfL9Q+4860uqS8f@gerhold.net/

Many thanks for such informative clarification, especially for
pointing me to  the rpmsg_wwan_ctrl driver. I saw it, but by a some
reason I did not link it to BAM-DMUX. Reading these links in
conjunction with your parallel talks make the situation much more
clear. I could not say that "I know kung fu", but I can say that now I
know how complex kung fu is.

--
Sergey