Re: [PATCH v1 4/4] USB: gadget: ACM: don't announce V.25ter support

From: Ahmad Fatoum <a.fatoum@pengutronix.de>
To: Ahmad Fatoum <ahmad@a3f.at>, barebox@lists.infradead.org
Cc: bst@pengutronix.de
Subject: Re: [PATCH v1 4/4] USB: gadget: ACM: don't announce V.25ter support
Date: Sat, 21 Sep 2019 13:00:37 +0200	[thread overview]
Message-ID: <bd80b9e6-1290-8d48-2104-6a44aef18dad@pengutronix.de> (raw)
In-Reply-To: <ef9d7f48-944c-62e7-2dc0-76b2e97451da@pengutronix.de>

On 9/20/19 10:51 AM, Ahmad Fatoum wrote:
> On 9/20/19 9:58 AM, Ahmad Fatoum wrote:
>> barebox currently announces support for ITU V.25ter AT commands, but
>> doesn't handle them specially when they arrive. Instead they are passed
>> as is to the sole barebox input console, where it may interfere with
>> valid user input.
>> This is especially annoying as ModemManager probes ttyACM devices that
>> announce their AT command support. So even when not using the ttyACM
>> device at all, the other UART ports are affected.
>>
>> Fix this by ceasing to announce USB_CDC_ACM_PROTO_AT_V25TER as function
>> protocol. After applying this patch, I can't see any spurious AT or ~x~
>> symbols on the console anymore.
> 
> I've also looked into the possibility of barebox emulating a USB-Serial
> port and showing up as a ttyUSB device under Linux.
> Apparently the two most popular chips are FTDI's and Prolific's, but both
> have taken steps in the past to make their Windows drivers not work with
> cloned chips[1][2]. I figured the safest to emulate would be the chip
> used safely in these Chinese Arduino Knock-offs because they're so prevalent
> and stumbled upon the CH340. Its drivers are mainline for Linux, but for other
> platforms, they need to be downloaded manually.
> 
> Seeing that the change to 'fix' barebox ttyACM is just two lines, I settled
> for that instead. It might be possible to get rid of the control interface
> descriptor altogether, but I don't know whether it would impact the portability
> across operating systems, so I've opted for this less invasive patch.

Getting rid of the control descriptor makes Linux ttyACM disappear, so I would
stick to this patch.

> Only tested on Linux v5.2 so far.

Tested working as well on Windows 10 Home and macOS Mojave.

> 
> Cheers
> Ahmad
> 
> [1]: https://hackaday.com/2016/02/01/ftdi-drivers-break-fake-chips-again/
> [2]: https://www.eevblog.com/forum/reviews/ftdi-driver-kills-fake-ftdi-ft232/msg534439/#msg534439
> 
>>
>> Cc: <bst@pengutronix.de>
>> Cc: <jlu@pengutronix.de>
>> Signed-off-by: Ahmad Fatoum <ahmad@a3f.at>
>> ---
>>  drivers/usb/gadget/f_acm.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/usb/gadget/f_acm.c b/drivers/usb/gadget/f_acm.c
>> index cba59b15859b..42a2b03ad291 100644
>> --- a/drivers/usb/gadget/f_acm.c
>> +++ b/drivers/usb/gadget/f_acm.c
>> @@ -104,7 +104,7 @@ acm_iad_descriptor = {
>>  	.bInterfaceCount = 	2,	// control + data
>>  	.bFunctionClass =	USB_CLASS_COMM,
>>  	.bFunctionSubClass =	USB_CDC_SUBCLASS_ACM,
>> -	.bFunctionProtocol =	USB_CDC_ACM_PROTO_AT_V25TER,
>> +	.bFunctionProtocol =	USB_CDC_PROTO_NONE,
>>  	/* .iFunction =		DYNAMIC */
>>  };
>>  
>> @@ -116,7 +116,7 @@ static struct usb_interface_descriptor acm_control_interface_desc = {
>>  	.bNumEndpoints =	1,
>>  	.bInterfaceClass =	USB_CLASS_COMM,
>>  	.bInterfaceSubClass =	USB_CDC_SUBCLASS_ACM,
>> -	.bInterfaceProtocol =	USB_CDC_ACM_PROTO_AT_V25TER,
>> +	.bInterfaceProtocol =	USB_CDC_PROTO_NONE,
>>  	/* .iInterface = DYNAMIC */
>>  };
>>  
>>
> 

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