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    SubjectRe: [RESEND PATCH v18 0/3] userspace MHI client interface driver
    On Wed 10 Feb 12:41 CST 2021, Jakub Kicinski wrote:

    > On Wed, 10 Feb 2021 11:55:31 +0530 Manivannan Sadhasivam wrote:
    > > On Tue, Feb 09, 2021 at 08:17:44AM -0800, Jakub Kicinski wrote:
    > > > On Tue, 9 Feb 2021 10:20:30 +0100 Aleksander Morgado wrote:
    > > > > This may be a stupid suggestion, but would the integration look less a
    > > > > backdoor if it would have been named "mhi_wwan" and it exposed already
    > > > > all the AT+DIAG+QMI+MBIM+NMEA possible channels as chardevs, not just
    > > > > QMI?
    > > >
    > > > What's DIAG? Who's going to remember that this is a backdoor driver
    > > > a year from now when Qualcomm sends a one liner patches which just
    > > > adds a single ID to open another channel?
    > >
    > > I really appreciate your feedback on this driver eventhough I'm not
    > > inclined with you calling this driver a "backdoor interface". But can
    > > you please propose a solution on how to make this driver a good one as
    > > per your thoughts?
    > >
    > > I really don't know what bothers you even if the userspace tools making
    > > use of these chardevs are available openly (you can do the audit and see
    > > if anything wrong we are doing).
    >
    > What bothers me is maintaining shim drivers which just shuttle opaque
    > messages between user space and firmware. One of which definitely is,
    > and the other may well be, proprietary. This is an open source project,
    > users are supposed to be able to meaningfully change the behavior of
    > the system.
    >

    You're absolutely right in that we in general don't like shim drivers
    and there are several examples of proper MHI drivers - for e.g.
    networking, WiFi

    Technically we could fork/reimplement
    https://github.com/freedesktop/libqmi, https://github.com/andersson/diag
    and https://github.com/andersson/qdl in the kernel as "proper drivers" -
    each one exposing their own userspace ABI.

    But to leave these in userspace and rely on something that looks exactly
    like USBDEVFS seems like a much better strategy.

    > What bothers me is that we have 3 WWAN vendors all doing their own
    > thing and no common Linux API for WWAN. It may have been fine 10 years
    > ago, but WWAN is increasingly complex and important.
    >

    We had a deep discussion and a few prototypes for a WWAN framework going
    around 1-1.5 years ago. Unfortunately, what did fit Intel's view of what
    a WWAN device is didn't fit at all with what's run and exposed by the
    "modem" DSP in a Qualcomm platform. After trying to find various
    contrived ways to model this we gave up.

    > > And exposing the raw access to the
    > > hardware is not a new thing in kernel. There are several existing
    > > subsystems/drivers does this as pointed out by Bjorn. Moreover we don't
    > > have in-kernel APIs for the functionalities exposed by this driver and
    > > creating one is not feasible as explained by many.
    > >
    > > So please let us know the path forward on this series. We are open to
    > > any suggestions but you haven't provided one till now.
    >
    > Well. You sure know how to aggravate people. I said clearly that you
    > can move forward on purpose build drivers (e.g. for WWAN). There is no
    > way forward on this common shim driver as far as I'm concerned.

    But what is a WWAN device? What features does it have? What kind of APIs
    does it expose?


    Note that in this sense "QMI" really is a "binary equivalent" of AT
    commands, the data flows over a DMA engine, which is not part of the
    "WWAN device" and other services, such as GPS, already has specific
    transports available upstream.

    Regards,
    Bjorn

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