On Mon, Sep 26, 2011 at 3:57 PM, Oliver Neukum <oneukum@xxxxxxx> wrote:
> Am Montag, 26. September 2011, 07:45:40 schrieb Xiaofan Chen:
>
>> I was actually thinking about this yesterday. I recently ran across this
>> bit of code in Linux's cdc-acm.c:
>>
>> case USB_CDC_CALL_MANAGEMENT_TYPE:
>> call_management_function = buffer[3];
>> call_interface_num = buffer[4];
>> if ( (quirks & NOT_A_MODEM) == 0 &&
>> (call_management_function & 3) != 3)
>> dev_err(&intf->dev, "This device cannot do
>> calls on its own. It is not a modem.\n");
>> break;
>>
>> Is it really a standards compliant way to indicate a communication
>> class device is not a modem? It seems almost too good to be true!?
>
> Occasionally something good happens.
>
>> I pulled up a copy of the spec, and I believe the relevant part is
>> section 5.2.3.2 on page 34 to 35. Here is what the spec says
>> regarding that byte:
>>
>> The capabilities that this configuration
>> supports:
>>
>> D7..D2: RESERVED (Reset to zero)
>> D1: 0 - Device sends/receives call
>> management information only over
>> the Communication Class
>> interface.
>> 1 - Device can send/receive call
>> management information over a
>> Data Class interface.
>> D0: 0 - Device does not handle call
>> management itself.
>> 1 - Device handles call
>> management itself.
>>
>> The previous bits, in combination, identify
>> which call management scenario is used. If bit
>> D0 is reset to 0, then the value of bit D1 is
>> ignored. In this case, bit D1 is reset to zero for
>> future compatibility.
>>
>> So this does seem to say that if the byte is zero (or the lowest bit
>> is zero), the device does not handle call management, which seems
>> to mean it can not possibly be a modem. The Linux check should
>
> Yes.
>
>> probably only look at the lowest bit, since 0x01 would indicate a
>> modem that requires the call management ("AT" commands if ACM)
>> sent on the communication interface rather than the data interface
>> (which Linux's driver doesn't support).
>
> Exactly. Using the control endpoint is not a coding problem, as the necessary
> code is present in cdc-wdm, but how would you present this to user space.
> The assumption that payload and AT commands go through the same character
> device is built in pretty hard.
>
>> Then again, maybe this interpretation of the communication class
>> spec is colored by my desire for better usability for non-modem
>> devices?
>
> Not at all, this code path is rarely triggered, but I cannot remember
> false positives.
>
>> Could it really be this easy, just a single bit: 1 = modem,
>> 0 = something not a modem.
>
> In theory it is that simple.
>
>> Of course, if there's an ECM or other networking descriptor, then
>> the network prefs dialog should obviously apply. But if there's an
>> ACM descriptor and no ECM or other networking, and that bit is 0,
>> is that a good enough standards compliant way to avoid popping
>> up the network prefs which causes user confusion when the device
>> isn't a modem?
>
> The capability is already exported via sysfs
>
> /*
> * attributes exported through sysfs
> */
> static ssize_t show_caps
> (struct device *dev, struct device_attribute *attr, char *buf)
> {
> struct usb_interface *intf = to_usb_interface(dev);
> struct acm *acm = usb_get_intfdata(intf);
>
> return sprintf(buf, "%d", acm->ctrl_caps);
> }
> static DEVICE_ATTR(bmCapabilities, S_IRUGO, show_caps, NULL);
>
> User space just has to look.
>
> Regards
> Oliver
>
Thanks a lot for the detail explanations.
Now Paul has submitted this bug to Apple as Issue #10187399.
It is a long thread and Apple initially denied the request by various
CDC-ACM users. Once they see your response, I think they
are kind of convinced.
http://lists.apple.com/archives/usb/2011//Sep/msg00031.html
http://lists.apple.com/archives/usb/2011//Sep/msg00032.html
http://lists.apple.com/archives/usb/2011//Sep/msg00034.html
--
Xiaofan
--
To unsubscribe from this list: send the line "unsubscribe linux-usb" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
