On Tue, Apr 05, 2022 at 01:24:37PM +0300, Ilpo Järvinen wrote:
> When tty is not read from, XON/XOFF may get stuck into an
> intermediate buffer. As those characters are there to do software
> flow-control, it is not very useful. In the case where neither end
> reads from ttys, the receiving ends might not be able receive the
> XOFF characters and just keep sending more data to the opposite
> direction. This problem is almost guaranteed to occur with DMA
> which sends data in large chunks.
>
> If TTY is slow to process characters, that is, eats less than given
> amount in receive_buf, invoke lookahead for the rest of the chars
> to process potential XON/XOFF characters.
>
> The guards necessary for ensuring the XON/XOFF character are
> processed only once were added by the previous patch. All this patch
> needs to do on that front is to pass the lookahead count (that can
> now be non-zero) into port->client_ops->receive_buf().
...
> +static bool __n_tty_receive_char_special(struct tty_struct *tty, unsigned char c,
> + bool lookahead_done)
> +{
> + if (!I_IXON(tty))
> + return false;
> +
> + if (c == START_CHAR(tty)) {
> + if (!lookahead_done) {
> + start_tty(tty);
> + process_echoes(tty);
> + }
> + return true;
> + }
> + if (c == STOP_CHAR(tty)) {
> + if (!lookahead_done)
> + stop_tty(tty);
> + return true;
> + }
> + return false;
> +}
Looking into this I would first make a preparatory patch that splits out
current code into something like
static bool __n_tty_receive_char_special_no_lookahead(struct tty_struct *tty, unsigned char c)
{
...current code...
}
Then in the patch 1 you add
static bool __n_tty_receive_char_special_lookahead(struct tty_struct *tty, unsigned char c)
{
...
}
static bool __n_tty_receive_char_special(struct tty_struct *tty, unsigned char c,
bool lookahead_done)
{
if (!I_IXON(tty))
return false;
if (lookahead_done)
return _lookahead();
return _no_lookahead();
}
?
...
/*
* Also start multi-line comments with
* capital letter and finish with
* grammar period.
*/
--
With Best Regards,
Andy Shevchenko
