Re: [PATCH v2 2/2] usb: gadget serial: Honour termios CLOCAL on disconnect

["Kyösti Mälkki" <kyosti.malkki-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On 11/03/2014 05:33 PM, Peter Hurley wrote:
> Hi Kyösti,
>
> On 11/03/2014 10:18 AM, Kyösti Mälkki wrote:
>> There are applications where it is desirable to not hangup ttyGS* when
>> USB disconnect is detected. USB host side of communication may
>> power-cycle periodically or there may be the actual need to physically
>> disconnect and reconnect USB cable temporarily.
>>
>> USB disconnects on serial gadget are comparable to loss of Carrier Detect
>> of conventional UARTs. With the change, if ttyGS* has termios CLOCAL flag
>> set, disconnect on USB does not hangup the TTY.
>>
>> Signed-off-by: Kyösti Mälkki <kyosti.malkki-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
>> ---
>>   drivers/usb/gadget/function/u_serial.c | 11 +++++++++--
>>   1 file changed, 9 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/usb/gadget/function/u_serial.c b/drivers/usb/gadget/function/u_serial.c
>> index 491082a..dabc165 100644
>> --- a/drivers/usb/gadget/function/u_serial.c
>> +++ b/drivers/usb/gadget/function/u_serial.c
>> @@ -1253,9 +1253,16 @@ void gserial_disconnect(struct gserial *gser)
>>   	port->port_usb = NULL;
>>   	gser->ioport = NULL;
>>   	if (port->port.count > 0 || port->openclose) {
>> +		struct tty_struct *tty;
>> +
>>   		wake_up_interruptible(&port->drain_wait);
>> -		if (port->port.tty)
>> -			tty_hangup(port->port.tty);
>> +		tty = port->port.tty;
>> +		if (tty) {
>> +			if (tty->termios.c_cflag & CLOCAL)
>> +				stop_tty(tty);
>> +			else
>> +				tty_hangup(tty);
>
> It seems you missed my earlier email: what happens if you leave
> out the stop_tty() call here?
>

Hi Peter

I did not miss it. My intro to the patch series says I am not familiar 
with tty infra, so I did not realize the question was directed to me.
Also my setup with serial gadget runs kernel 3.12.6 so I cannot really 
tell or test if tty changes since that would make my patch invalid or 
unnecessary.

In the previous change for gserial_disconnect(), where the tty_hangup() 
call was added, commit message described it fixes an endless loop on tty 
device read and select. Would we be back at that situation without call 
to stop_tty()?

> I ask because the tty is still restartable from userspace after you
> stop_tty() here. So if your goal is to prevent write() from
> happening, this won't work.

There was no goal to prevent write() on the tty device, should there be? 
Or should it silently just drop any characters while in the disconnected 
state instead of buffering (some of) them?

Thanks,
KM

>
> Regards,
> Peter Hurley
>
>> +		}
>>   	}
>>   	spin_unlock_irqrestore(&port->port_lock, flags);
>>
>>
>

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