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authorIlpo Järvinen <ilpo.jarvinen@linux.intel.com>2022-06-06 18:36:51 +0300
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2022-06-10 14:51:31 +0300
commit6bb6fa6908ebd3cb4e14cd4f0ce272ec885d2eb0 (patch)
tree9d29bb37457ad995881181e024b93bc968a0dd30 /include/linux/tty_buffer.h
parent84f2faa7852e1f55d89bb0c99b3a672b87b11f87 (diff)
downloadlinux-6bb6fa6908ebd3cb4e14cd4f0ce272ec885d2eb0.tar.xz
tty: Implement lookahead to process XON/XOFF timely
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. We need to keep track of how many characters have been processed by the lookahead to avoid processing the flow control char again on the normal path. Bookkeeping occurs parallel on two layers (tty_buffer and n_tty) to avoid passing the lookahead_count through the whole call chain. When a flow-control char is processed, two things must occur: a) it must not be treated as normal char b) if not yet processed, flow-control actions need to be taken The return value of n_tty_receive_char_flow_ctrl() tells caller a), and b) is kept internal to n_tty_receive_char_flow_ctrl(). If characters were previous looked ahead, __receive_buf() makes two calls to the appropriate n_tty_receive_buf_* function. First call is made with lookahead_done=true for the characters that were subject to lookahead earlier and then with lookahead=false for the new characters. Either of the calls might be skipped when it has no characters to handle. Reported-by: Gilles Buloz <gilles.buloz@kontron.com> Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com> Link: https://lore.kernel.org/r/20220606153652.63554-2-ilpo.jarvinen@linux.intel.com Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'include/linux/tty_buffer.h')
-rw-r--r--include/linux/tty_buffer.h1
1 files changed, 1 insertions, 0 deletions
diff --git a/include/linux/tty_buffer.h b/include/linux/tty_buffer.h
index 3b9d77604291..1796648c2907 100644
--- a/include/linux/tty_buffer.h
+++ b/include/linux/tty_buffer.h
@@ -15,6 +15,7 @@ struct tty_buffer {
int used;
int size;
int commit;
+ int lookahead; /* Lazy update on recv, can become less than "read" */
int read;
int flags;
/* Data points here */