Re: Pty is losing bytes

[Sergey Vlasov <vsu@altlinux.ru>]

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On Tue, 15 Feb 2005 11:08:07 -0800 (PST) Linus Torvalds wrote:

> On Tue, 15 Feb 2005, Andreas Schwab wrote:
> >
> > Recent kernel are losing bytes on a pty. 
> 
> Great catch.
> 
> I think it may be a n_tty line discipline bug, brought on by the fact that
> the PTY buffering is now 4kB rather than 2kB. 4kB is also the
> N_TTY_BUF_SIZE, and if n_tty has some off-by-one error, that would explain 
> it.
> 
> Does the problem go away if you change the default value of "chunk" (in 
> drivers/char/tty_io.c:do_tty_write) from 4096 to 2048? If so, that means 
> that the pty code has _claimed_ to have written 4kB, and only ever wrote 
> 4kB-1 bytes. That in turn implies that "ldisc.receive_room()" disagrees 
> with "ldisc.receive_buf()".

The problem also goes away after unsetting ECHO on the slave terminal.
This seems to point to this code in n_tty_receive_char():

	if (L_ECHO(tty)) {
		if (tty->read_cnt >= N_TTY_BUF_SIZE-1) {
			put_char('\a', tty); /* beep if no space */
			return;
		}
	.......
	}

This code sets the maximum number of buffered characters to
N_TTY_BUF_SIZE-1, however, put_tty_queue() considers the maximum to be
N_TTY_BUF_SIZE, and n_tty_receive_room() also returns N_TTY_BUF_SIZE for
canonical mode if the canon_data buffer is empty - therefore after
unsetting ECHO bytes are no longer lost.

BTW, for the noncanonical mode n_tty_receive_room() calculates the
result assuming that the buffer can hold at most N_TTY_BUF_SIZE-1
characters - not the full N_TTY_BUF_SIZE.

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