Messages in this thread | | | Date | Tue, 5 Apr 2022 19:11:04 +0300 | From | Andy Shevchenko <> | Subject | Re: [PATCH 2/2] tty: Implement lookahead to process XON/XOFF timely |
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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
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