Messages in this thread | | | Date | Wed, 6 Apr 2022 10:21:12 +0200 | Subject | Re: [PATCH 2/2] tty: Implement lookahead to process XON/XOFF timely | From | Jiri Slaby <> |
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On 05. 04. 22, 18:11, Andy Shevchenko wrote: > 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)
This should be dubbed better. Maybe n_tty_receive_char_flow_control()?
And I would place the if (I_IXON(tty)) to the caller. I am a bit lost in this pseudo code, so maybe this doesn't make sense in your proposal. I have something like in my mind:
if (I_IXON(tty)) return n_tty_receive_char_flow_control();
Historically, these n_tty_receive* function names were a big mess. Don't produce more of that by simply prepending only "__".
> { > if (!I_IXON(tty)) > return false; > > if (lookahead_done) > return _lookahead(); > > return _no_lookahead(); > }
thanks -- js suse labs
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