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From: Felipe Balbi <>
To: "Andrew P. Lentvorski" <>,
	Alan Stern <>
Subject: Re: Unable to set "iInterface" in usb gadget via configfs
Date: Thu, 16 Jan 2020 15:02:12 +0200
Message-ID: <> (raw)
In-Reply-To: <>

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"Andrew P. Lentvorski" <> 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

>> 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.


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  reply index

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-14 23:58 Andrew P. Lentvorski
2020-01-15 15:14 ` Alan Stern
2020-01-16  2:23   ` Andrew P. Lentvorski
2020-01-16 13:02     ` Felipe Balbi [this message]
2020-01-17  0:39       ` Andrew P. Lentvorski
2020-01-17  9:25         ` Felipe Balbi
2020-01-18  0:58           ` Andrew P. Lentvorski
2020-01-19 16:45             ` Felipe Balbi

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