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From: "Bjørn Mork" <bjorn@mork.no>
To: "Stefan Brüns" <stefan.bruens@rwth-aachen.de>
Cc: Johan Hovold <johan@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	<linux-usb@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
	Daniele Palmas <dnlplm@gmail.com>
Subject: Re: [PATCH v2] USB: serial: qcserial: Support for SDX55 based Sierra Wireless 5G modules
Date: Fri, 02 Jul 2021 09:32:28 +0200	[thread overview]
Message-ID: <87y2apf9wz.fsf@miraculix.mork.no> (raw)
In-Reply-To: <1727850.UKLhYeRy6v@pebbles> ("Stefan =?utf-8?Q?Br=C3=BCns=22?= =?utf-8?Q?'s?= message of "Thu, 1 Jul 2021 18:28:14 +0200")

Stefan Brüns <stefan.bruens@rwth-aachen.de> writes:

> On Donnerstag, 24. Juni 2021 13:15:07 CEST Bjørn Mork wrote:
>> Johan Hovold <johan@kernel.org> writes:
>> > Could you please also post the output of usb-devices (or lsusb -v) for
>> > this device in MBIM mode?
>> 
>> Yes, this would be nice to have.
>
> See below.
>  
>> I suspect that this device is like other SDX55 devices we've seen, using
>> class/subclass/function to map the vendor specific functions
>> too. Dropping static interface numbers.  If correct, then the patch is
>> bogus and the interface numbers might change based on firmware version
>> and configuration.
>
> Do you really expect Sierra do to something sensible? According to their 
> documentation functions are matched by interface numbers.

Well... I expect them to be as sensible as any other vendor.  And I
expect docs to be intendend as guidance only ;-)

> They still use broken interface descriptors with holes in interface numbering 
> (i.e. interface number 2 does not exist, which violates the USB standard).

Right. Wrt the violation, I think that train left a decade ago.  

>     Interface Descriptor:
>       bLength                 9
>       bDescriptorType         4
>       bInterfaceNumber        3
>       bAlternateSetting       0
>       bNumEndpoints           3
>       bInterfaceClass       255 Vendor Specific Class
>       bInterfaceSubClass      0 
>       bInterfaceProtocol      0 
[..]
>     Interface Descriptor:
>       bLength                 9
>       bDescriptorType         4
>       bInterfaceNumber        4
>       bAlternateSetting       0
>       bNumEndpoints           2
>       bInterfaceClass       255 Vendor Specific Class
>       bInterfaceSubClass    255 Vendor Specific Subclass
>       bInterfaceProtocol     48 


So Sierra do follow the same pattern we've seen on other X55 devices:

ff/00/00 - AT
ff/ff/30 - QCDM

See commits

 accf227de4d2 ("USB: serial: option: Add support for Quectel RM500Q")
 d6c1ddd938d8 ("USB: serial: option: add Quectel EM160R-GL")

for other examples.

This obviously doesn't make any difference if your configuration is the
only one.  But I believe that is unlikely.  There are probably ways the
layout can be changed, even if currenly not documented.  The advantage
of class/subclass/protocol matching to function is that it works
regardless of the number of functions and their interface number.


Bjørn

  reply	other threads:[~2021-07-02  7:32 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-11 13:45 [PATCH] Add support for SDX55 based Sierra Wireless 5G modules Stefan Brüns
2021-06-11 13:58 ` [PATCH v2] USB: serial: qcserial: Support " Stefan Brüns
2021-06-24  7:28   ` Johan Hovold
2021-06-24 11:15     ` Bjørn Mork
2021-07-01 16:28       ` Stefan Brüns
2021-07-02  7:32         ` Bjørn Mork [this message]
2021-07-01 16:41     ` Stefan Brüns
2021-07-02  8:27       ` Johan Hovold

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