Re: Unable to set "iInterface" in usb gadget via configfs

[Felipe Balbi <balbi@kernel.org>]

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hi,

"Andrew P. Lentvorski" <bsder@allcaps.org> writes:
> On 1/15/20 7:14 AM, Alan Stern wrote:
>
>> I don't think any earlier messages in this thread made it through the 
>> mailing list, but this one definitely did.
>
> Yay!  I also saw this one in the archives so I was hopeful.
>
>>>> I've been trying to set "iInterface" in my usb gadget to a specific
>>>> string, but I simply can't find a way to make configfs accept this.

iInterface is not a string. It's the index to a string descriptor table.

>>>> When I set my gadget up on my Beaglebone Black (uname -a: Linux
>>>> beaglebone 4.14.108-ti-r113 #1 SMP PREEMPT Wed Jul 31 00:01:10 UTC
>>>> 2019 armv7l GNU/Linux).
>>>>
>>>> I get (output from lsusb):
>>>>
>>>> iInterface 5 HID Interface
>>>>
>>>>
>>>> But I want it to be something like:
>>>>
>>>> iInterface 4 LPC-LINK2 CMSIS-DAP V5.224

Why? Oh, you want your beaglebone to behave as a CMSIS-DAP to trick some
other SW?

Do we need to support that upstream, though? It seems like this is a
one-off thing. Does anybody else need to configure interface string
descriptor?

>> Then maybe you need to fix f_hid.c.  Or maybe configfs isn't meant to
>> allow the user to specify these string index values (I don't know any
>> of the configfs details).
>
> That's kind of my problem in that I was hoping to get someone far more
> knowledegable than me to at least flag these before attacking it:
>
> A) I didn't overlook something stupid and this really is hardwired with
> no way to change it (either in configfs or ... some other? kernel mechanism)

yes, it really is hardwired

> B) This is an *actual* bug.

Not a bug, just was never a requirement.

> C) This is an actual bug *that should be fixed* and isn't that way
> intentionally for some Linux reason.

Up until now, it has been intentional. Currently, I don't see a need to
change it. I may be persuaded otherwise, but I need to see arguments
other than "I want to trick some SW into thinking I'm something else".

> D) This is actually the right place to fix it.  Obviously there is going
> to be something at the configfs level, too, and I have *zero* idea where
> to start looking for that.

drivers/usb/gadget/configfs.c, look for GS_STRINGS_RW(). Note, also,
that you should *not* allow for strings to be changed after the device
has been enumerated.

-- 
balbi

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