Re: Unsupported device message, Compliance C.2, part 4

Alan Stern wrote:
> On Fri, 5 Oct 2007, Aras Vaichas wrote:
>> Hi,
>>
>> We need to comply with the "USB-IF Embedded Host Compliance Plan", section C.2, 
>> part 4 which states:
>>
>> "4. Messaging (No Silent Failure):
>> 	a. Unsupported device message. Attach a device that is known not to be 
>> supported. An appropriate message should be displayed.
>> 	b. If hubs are not supported, a "Hubs not supported" or similar message must 
>> appear. A generic "device not supported" message is not acceptable on a hub 
>> connect."
> 
> Is this meant to apply to all USB hosts?  Only "embedded" hosts 
> (whatever they are)?  Only OTG devices?


The compliance plan document defines an Embedded Host as: "A USB device with 
limited hardware resources. An embedded host's capability is application 
specific with no ability (or restricted ability) to dynamically add new device 
support."

see http://www.usb.org/developers/docs/EH_Compliance_v1_0.pdf


Further clarification of "No Silent Failure" can be found here:

http://compliance.usb.org/index.asp?UpdateFile=Policies&Format=Standard#13

"The USB-IF enforces a "no silent failure" rule. This means that an 
implementation of USB must not appear broken to the consumer. In configurations 
where the consumer's expectations are not met, either the peripheral or host 
must provide appropriate and useful feedback to the consumer regarding the 
problem."


> The requirements are not entirely clear.  What does it mean for a
> device to be supported?  Is it supported if there is a driver compiled
> into the kernel?  What if there is a driver in a loadable module?  
> What if there is a userspace driver?  

It is supported if the USB device can be plugged into the host and will 
function as expected.

I looked at it this way, if I've created a system where I only have device 
driver support for USB Flash Drives, and I plug in a USB Serial Dongle, the 
system should create a log message which says something like, "Unsupported USB 
device inserted".


> What if there is a driver for
> some of a device's interfaces but not all of them?

Then it should not fail those interfaces silently.


> Also, what does it mean for a device to be "known" not to be supported?  
> Are there any USB devices "known" not be supported by Linux?

Taking the context of an embedded system, everything it knows is held within 
its file system. If the filing system doesn't hold an information about the 
device, then it is unknown.


> Part b shouldn't matter; hub support is built into the Linux USB core.
> 
>> Does anyone have any thoughts on how I would go about this? Has anyone else had 
>> to deal with this?
>>
>> I was thinking that it might well be a udev question, but as it's USB 
>> compliance related I thought I would start on this list.
> 
> The requirements are so fuzzy, I don't see how anyone could ever tell
> whether they were fulfilled or not.  Unless it's for something like
> OTG, where there really is a whitelist of supported devices.

Well we have to come to some conclusion on this compliance question if we want 
Linux devices to be officially USB certified.

I've seen postings like "USB device not claimed by an active driver"
e.g. http://linux.derkeiler.com/Mailing-Lists/RedHat/2004-06/0287.html

I figure this is what is needed, but I can't find the code that produces this 
message!


Aras

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