Messages in this thread |  | | Date | Tue, 17 Feb 2015 14:03:58 -0500 | From | Peter Hurley <> | Subject | Re: [PATCH 1/1] pty: BREAK for pseudoterminals |
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On 02/16/2015 08:04 AM, Peter Hurley wrote: > On 02/05/2015 02:11 PM, Nan Li wrote: >> This will greatly enhance the usefulness of QEMU virtual serial ports, because the Linux kernel interprets a break on the serial console as a SysRq, but there is currently no way to pass this signal over a pseudo-terminal. This patch will work for transmitting BREAK from master to slave through pseudo-terminal. > > pty is an IPC mechanism, not a virtualization driver. > > Those programs that know they're consuming from a slave pty may not > have bothered to set termios for BREAKs, and may not be prepared > for NULLs in the input stream.
I'm not even seeing how this works as advertised.
In QEMU, the *master* end is the source/sink for the guest VM, while the *slave* pty is sink/source for the host. Like this:
Host <--> slave pty <===> master pty <--> qemu <==> emulated serial port
So this patch enables the guest VM to transmit BREAK to the host. Why is that useful?
Regards, Peter Hurley
>> Signed-off-by: Nan Li <nli@suse.com> >> --- >> drivers/tty/pty.c | 19 +++++++++++++++++++ >> 1 file changed, 19 insertions(+) >> >> diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c >> index e72ee62..ac8893a 100644 >> --- a/drivers/tty/pty.c >> +++ b/drivers/tty/pty.c >> @@ -31,6 +31,7 @@ >> static struct tty_driver *ptm_driver; >> static struct tty_driver *pts_driver; >> static DEFINE_MUTEX(devpts_mutex); >> +#define BREAK_STRING '\0' >> #endif >> >> static void pty_close(struct tty_struct *tty, struct file *filp) >> @@ -674,6 +675,23 @@ static void pty_unix98_shutdown(struct tty_struct *tty) >> devpts_kill_index(tty->driver_data, tty->index); >> } >> >> +static int pty_unix98_break_ctl(struct tty_struct *tty, int break_state) >> +{ >> + int c; >> + struct tty_struct *to = tty->link; >> + >> + if (break_state) { >> + /* Stuff the break data into the input queue of the other end */ >> + c = tty_insert_flip_char(to->port, BREAK_STRING, TTY_BREAK); >> + /* And shovel */ >> + if (c) >> + tty_flip_buffer_push(to->port); >> + else >> + return -ENOMEM; >> + } >> + return 0; >> +} >> + >> static const struct tty_operations ptm_unix98_ops = { >> .lookup = ptm_unix98_lookup, >> .install = pty_unix98_install, >> @@ -686,6 +704,7 @@ static const struct tty_operations ptm_unix98_ops = { >> .chars_in_buffer = pty_chars_in_buffer, >> .unthrottle = pty_unthrottle, >> .ioctl = pty_unix98_ioctl, >> + .break_ctl = pty_unix98_break_ctl, >> .resize = pty_resize, >> .shutdown = pty_unix98_shutdown, >> .cleanup = pty_cleanup >> >
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