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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>
60 lines
1.3 KiB
C
60 lines
1.3 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef _LINUX_TTY_BUFFER_H
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#define _LINUX_TTY_BUFFER_H
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#include <linux/atomic.h>
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#include <linux/llist.h>
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#include <linux/mutex.h>
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#include <linux/workqueue.h>
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struct tty_buffer {
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union {
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struct tty_buffer *next;
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struct llist_node free;
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};
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int used;
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int size;
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int commit;
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int lookahead; /* Lazy update on recv, can become less than "read" */
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int read;
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int flags;
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/* Data points here */
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unsigned long data[];
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};
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/* Values for .flags field of tty_buffer */
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#define TTYB_NORMAL 1 /* buffer has no flags buffer */
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static inline unsigned char *char_buf_ptr(struct tty_buffer *b, int ofs)
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{
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return ((unsigned char *)b->data) + ofs;
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}
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static inline char *flag_buf_ptr(struct tty_buffer *b, int ofs)
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{
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return (char *)char_buf_ptr(b, ofs) + b->size;
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}
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struct tty_bufhead {
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struct tty_buffer *head; /* Queue head */
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struct work_struct work;
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struct mutex lock;
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atomic_t priority;
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struct tty_buffer sentinel;
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struct llist_head free; /* Free queue head */
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atomic_t mem_used; /* In-use buffers excluding free list */
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int mem_limit;
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struct tty_buffer *tail; /* Active buffer */
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};
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/*
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* When a break, frame error, or parity error happens, these codes are
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* stuffed into the flags buffer.
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*/
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#define TTY_NORMAL 0
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#define TTY_BREAK 1
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#define TTY_FRAME 2
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#define TTY_PARITY 3
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#define TTY_OVERRUN 4
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#endif
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