linux-kernel - Re: [RFC PATCH net-next 0/4] net: wwan: Add Qualcomm BAM-DMUX WWAN network driver

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Date:   Mon, 19 Jul 2021 09:43:27 -0600
From:   Jeffrey Hugo <jeffrey.l.hugo@...il.com>
To:     Stephan Gerhold <stephan@...hold.net>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Loic Poulain <loic.poulain@...aro.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>,
        "open list:NETWORKING DRIVERS" <netdev@...r.kernel.org>,
        MSM <linux-arm-msm@...r.kernel.org>, dmaengine@...r.kernel.org,
        DTML <devicetree@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>, phone-devel@...r.kernel.org,
        ~postmarketos/upstreaming@...ts.sr.ht
Subject: Re: [RFC PATCH net-next 0/4] net: wwan: Add Qualcomm BAM-DMUX WWAN
 network driver

On Mon, Jul 19, 2021 at 9:01 AM Stephan Gerhold <stephan@...hold.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. This series adds a driver that allows using it.
>
> For more information about BAM-DMUX, see PATCH 4/4.
>
> Shortly said, BAM-DMUX is built using a simple protocol layer on top of
> a DMA engine (Qualcomm BAM DMA). For BAM-DMUX, the BAM DMA engine runs in
> a quite strange mode that I call "remote power collapse", where the
> modem/remote side is responsible for powering on the BAM when needed but we
> are responsible to initialize it. The BAM is power-collapsed when unneeded
> by coordinating power control via bidirectional interrupts from the
> BAM-DMUX driver.

The hardware is physically located on the modem, and tied to the modem
regulators, etc.  The modem has the ultimate "off" switch.  However,
due to the BAM architecture (which is complicated), configuration uses
cooperation on both ends.

>
> The series first adds one possible solution for handling this "remote power
> collapse" mode in the bam_dma driver, then it adds the BAM-DMUX driver to
> the WWAN subsystem. Note that the BAM-DMUX driver does not actually make
> use of the WWAN subsystem yet, since I'm not sure how to fit it in there
> yet (see PATCH 4/4).
>
> Please note that all of the changes in this patch series are based on
> a fairly complicated driver from Qualcomm [1].
> I do not have access to any documentation about "BAM-DMUX". :(

I'm pretty sure I still have the internal docs.

Are there specific things you want to know?