Re: Default ECHO on TTYs causes unwanted garbage chars

["gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>]

On Tue, Jun 09, 2020 at 01:13:06PM +0000, Joakim Tjernlund wrote:
> On Tue, 2020-06-09 at 13:57 +0200, Greg KH wrote:
> > 
> > On Tue, Jun 09, 2020 at 11:38:49AM +0000, Joakim Tjernlund wrote:
> > > Hi List
> > > 
> > > I was advised to come here with this problem(started on the USB list).
> > > 
> > > We have a USB to RS232 bridge which presents itself as an ttyACM and the first connect after power on,
> > > we see some garbage chars transmitted back from USB host(PC) to out device which becomes input to
> > > the device.
> > > 
> > > After much debugging I found that this are chars sent early in the boot process which then
> > > are buffered and the TTYs default to ECHO chars is the cause.
> > 
> > So some program in the boot sequence is trying to send data out the
> > device?  Why not just not do that?
> 
> This is the boot console. Both u-boot and Linux prints a lot there, then init prints while starting services

So the same device is used for boot console as well as a ttyACM device
later on?

> > > When the TTY is opened, any chars in the this buffer is ECHOed back over USB to the device,
> > > before one has a chance to disable ECHO. The device then thinks these chars are regular input.
> > 
> > Wait, you said something in the boot process did write to the device,
> > which would have caused the tty to be opened then, right?
> 
> well, boot process of the device prints and it is enough for the USB cable to be attached but not opened by any app.
> The device just see an UART and prints when UART is initialized.

What tool does that?  Why not fix that?

> > > Seems to me that this behaviour is unwanted in general and and app. should get a chance to flush/discard
> > > any chars so this does not happen.
> > 
> > Where are the characters coming from that would need to be flushed?
> 
> Early output from boot, basically whatever prints just after connecting the USB cable.

Then don't have boot print to that device :)

> > When should characters be flushed exactly?
> 
> Whatever is in the buffers before opening the tty.

But what is supposed to happen to the data that was sent to it while it
was "closed"?

> The terminal app(like cu) tries to flush any input when it starts, just to avoid any old chars in the
> queue but it is to late then.

I strongly just suggest having userspace not write to the device to
start with, that would solve this, right?

thanks,

greg k-h