• Bjørn Mork's avatar
    cdc_ether: fix rndis support for Mediatek based smartphones · 4d7ffcf3
    Bjørn Mork authored
    A Mediatek based smartphone owner reports problems with USB
    tethering in Linux.  The verbose USB listing shows a rndis_host
    interface pair (e0/01/03 + 10/00/00), but the driver fails to
    bind with
    
    [  355.960428] usb 1-4: bad CDC descriptors
    
    The problem is a failsafe test intended to filter out ACM serial
    functions using the same 02/02/ff class/subclass/protocol as RNDIS.
    The serial functions are recognized by their non-zero bmCapabilities.
    
    No RNDIS function with non-zero bmCapabilities were known at the time
    this failsafe was added. But it turns out that some Wireless class
    RNDIS functions are using the bmCapabilities field. These functions
    are uniquely identified as RNDIS by their class/subclass/protocol, so
    the failing test can safely be disabled.  The same applies to the two
    types of Misc class RNDIS functions.
    
    Applying the failsafe to Communication class functions only retains
    the original functionality, and fixes the problem for the Mediatek based
    smartphone.
    
    Tow examples of CDC functional descriptors with non-zero bmCapabilities
    from Wireless class RNDIS functions are:
    
    0e8d:000a  Mediatek Crosscall Spider X5 3G Phone
    
          CDC Header:
            bcdCDC               1.10
          CDC ACM:
            bmCapabilities       0x0f
              connection notifications
              sends break
              line coding and serial state
              get/set/clear comm features
          CDC Union:
            bMasterInterface        0
            bSlaveInterface         1
          CDC Call Management:
            bmCapabilities       0x03
              call management
              use DataInterface
            bDataInterface          1
    
    and
    
    19d2:1023  ZTE K4201-z
    
          CDC Header:
            bcdCDC               1.10
          CDC ACM:
            bmCapabilities       0x02
              line coding and serial state
          CDC Call Management:
            bmCapabilities       0x03
              call management
              use DataInterface
            bDataInterface          1
          CDC Union:
            bMasterInterface        0
            bSlaveInterface         1
    
    The Mediatek example is believed to apply to most smartphones with
    Mediatek firmware.  The ZTE example is most likely also part of a larger
    family of devices/firmwares.
    Suggested-by: default avatarLars Melin <larsm17@gmail.com>
    Signed-off-by: default avatarBjørn Mork <bjorn@mork.no>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    4d7ffcf3
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