Re: [PATCH] cdc-acm: acm_init: Set initial BAUD to B0

[Greg KH <greg@kroah.com>]

On Fri, Jul 10, 2020 at 10:16:33AM +0000, Joakim Tjernlund wrote:
> On Fri, 2020-07-10 at 11:54 +0200, Johan Hovold wrote:
> > CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you recognize the sender and know the content is safe.
> > 
> > 
> > On Fri, Jul 10, 2020 at 11:35:18AM +0200, Joakim Tjernlund wrote:
> > > BO will disable USB input until the device opens. This will
> > > avoid garbage chars waiting flood the TTY. This mimics a real UART
> > > device better.
> > > For initial termios to reach USB core, USB driver has to be
> > > registered before TTY driver.
> > > 
> > > Signed-off-by: Joakim Tjernlund <joakim.tjernlund@infinera.com>
> > > Cc: stable@vger.kernel.org
> > > ---
> > > 
> > >  I hope this change makes sense to you, if so I belive
> > >  ttyUSB could do the same.
> > 
> > No, this doesn't make sense. B0 is used to hang up an already open tty.
> 
> This is at module init so there is no tty yet.
> acm_probe() will later set:
>         acm->line.dwDTERate = cpu_to_le32(9600);
> 	acm->line.bDataBits = 8;
> 	acm_set_line(acm, &acm->line);
> 
> > 
> > Furthermore, this change only affects the initial terminal settings and
> > won't have any effect the next time you open the same port.
> 
> hmm, it is not ideal but acm_probe() will later set:
>         acm->line.dwDTERate = cpu_to_le32(9600);
> 	acm->line.bDataBits = 8;
> 	acm_set_line(acm, &acm->line);
> 
> But, would it not make sense to not accept input until TTY is opened ?
> That would be more inline with a real RS232, would it not?

You can't keep the chip in the usb-to-serial device from accepting data
before you do anything with it, that requires firmware to not do this.
Are you sure your firmware is correct?

> > Why not fix your firmware so that it doesn't transmit before DTR is
> > asserted during open()?
> 
> I would but it is not my firmware, it is a ready made USB to UART chip. will talk
> to the manufacturer though.

What chip is this?

thanks,

greg k-h