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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>
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Define a kfifo inside struct tty_port. We use DECLARE_KFIFO_PTR and let
the preexisting tty_port::xmit_buf be also the buffer for the kfifo.
And handle the initialization/decomissioning along with xmit_buf, i.e.
in tty_port_alloc_xmit_buf() and tty_port_free_xmit_buf(), respectively.
This allows for kfifo use in drivers which opt-in, while others still
may use the old xmit_buf. mxser will be the first user in the next
few patches.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220124071430.14907-4-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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tty_port_operations used to have only comments along its members.
Convert them into proper kernel-doc comments in front of the structure.
And add some more explanation to them where needed.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20211126081611.11001-6-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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tty_port used to have only short comments along its members. Convert
them into proper kernel-doc comments in front of the structure. And add
some more explanation to them where needed.
The whole structure purpose and handling is documented at the end too --
some pieces of preexisting text moved to this place.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20211126081611.11001-3-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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tty.h is long enough already. And I am slowly adding kernel-doc
documentation, so it grows to unmaintainable long mess. To avoid this,
split tty.h further into tty_port.h and move there tty_port-related
declarations and function prototypes (those implemented in tty_port.c).
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210723103147.18250-8-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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Ilpo Järvinen
Jiri Slaby