M. Feser KBS GmbH Date: 10.01.2025 This patch modifies the TTY implementation to make use of a kthread for low latency configuration Modifications: reintroduced low_latency flag which is missing in newer kernel versions adjusted line numbers adjusted prio Based on the following patch series from Steven Walter (08.06.2015): drivers/tty: refactor functions for flushing/queuing work drivers/tty: convert tty_port to use kthread_worker drivers/tty/tty_buffer.c: use a separate kthread for low_latency ------------------------------------------------------------------------- diff -ruPN a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c --- a/drivers/tty/serial/serial_core.c 2025-01-09 13:28:51.000000000 +0100 +++ b/drivers/tty/serial/serial_core.c 2025-01-10 14:31:12.665776943 +0100 @@ -974,6 +974,7 @@ port->closing_wait = closing_wait; if (new_info->xmit_fifo_size) uport->fifosize = new_info->xmit_fifo_size; + port->low_latency = (uport->flags & UPF_LOW_LATENCY) ? 1 : 0; check_and_exit: retval = 0; diff -ruPN a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c --- a/drivers/tty/tty_buffer.c 2025-01-09 13:28:51.000000000 +0100 +++ b/drivers/tty/tty_buffer.c 2025-01-10 14:35:59.971952999 +0100 @@ -5,6 +5,7 @@ #include #include +#include #include #include #include @@ -12,6 +13,8 @@ #include #include #include +#include +#include #include #include #include @@ -72,7 +75,7 @@ atomic_dec(&buf->priority); mutex_unlock(&buf->lock); if (restart) - queue_work(system_unbound_wq, &buf->work); + tty_buffer_restart_work(port); } EXPORT_SYMBOL_GPL(tty_buffer_unlock_exclusive); @@ -487,7 +490,7 @@ * 'consumer' */ -static void flush_to_ldisc(struct work_struct *work) +static void flush_to_ldisc(struct kthread_work *work) { struct tty_port *port = container_of(work, struct tty_port, buf.work); struct tty_bufhead *buf = &port->buf; @@ -558,7 +561,7 @@ struct tty_bufhead *buf = &port->buf; tty_flip_buffer_commit(buf->tail); - queue_work(system_unbound_wq, &buf->work); + tty_buffer_restart_work(port); } EXPORT_SYMBOL(tty_flip_buffer_push); @@ -588,11 +591,25 @@ tty_flip_buffer_commit(buf->tail); spin_unlock_irqrestore(&port->lock, flags); - queue_work(system_unbound_wq, &buf->work); + tty_buffer_restart_work(port); return size; } +static DEFINE_KTHREAD_WORKER(tty_buffer_worker); +static DEFINE_KTHREAD_WORKER(tty_buffer_worker_ll); + +void tty_buffer_init_kthread() +{ + struct task_struct *task; + struct sched_param param = { .sched_priority = MAX_RT_PRIO/4 }; + + kthread_run(kthread_worker_fn, &tty_buffer_worker, "tty"); + task = kthread_run(kthread_worker_fn, &tty_buffer_worker_ll, + "tty-low-latency"); + sched_setscheduler(task, SCHED_FIFO, ¶m); +} + /** * tty_buffer_init - prepare a tty buffer structure * @port: tty port to initialise @@ -612,7 +629,7 @@ init_llist_head(&buf->free); atomic_set(&buf->mem_used, 0); atomic_set(&buf->priority, 0); - INIT_WORK(&buf->work, flush_to_ldisc); + kthread_init_work(&buf->work, flush_to_ldisc); buf->mem_limit = TTYB_DEFAULT_MEM_LIMIT; } @@ -642,15 +659,18 @@ bool tty_buffer_restart_work(struct tty_port *port) { - return queue_work(system_unbound_wq, &port->buf.work); + if (port->low_latency) + return kthread_queue_work(&tty_buffer_worker_ll, &port->buf.work); + else + return kthread_queue_work(&tty_buffer_worker, &port->buf.work); } bool tty_buffer_cancel_work(struct tty_port *port) { - return cancel_work_sync(&port->buf.work); + return kthread_cancel_work_sync(&port->buf.work); } void tty_buffer_flush_work(struct tty_port *port) { - flush_work(&port->buf.work); + kthread_flush_work(&port->buf.work); } diff -ruPN a/drivers/tty/tty.h b/drivers/tty/tty.h --- a/drivers/tty/tty.h 2025-01-09 13:28:51.000000000 +0100 +++ b/drivers/tty/tty.h 2025-01-10 13:23:26.000000000 +0100 @@ -71,6 +71,7 @@ void tty_buffer_free_all(struct tty_port *port); void tty_buffer_flush(struct tty_struct *tty, struct tty_ldisc *ld); void tty_buffer_init(struct tty_port *port); +void tty_buffer_init_kthread(void); void tty_buffer_set_lock_subclass(struct tty_port *port); bool tty_buffer_restart_work(struct tty_port *port); bool tty_buffer_cancel_work(struct tty_port *port); diff -ruPN a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c --- a/drivers/tty/tty_io.c 2025-01-09 13:28:51.000000000 +0100 +++ b/drivers/tty/tty_io.c 2025-01-10 13:23:26.000000000 +0100 @@ -3614,6 +3614,7 @@ int __init tty_init(void) { tty_sysctl_init(); + tty_buffer_init_kthread(); cdev_init(&tty_cdev, &tty_fops); if (cdev_add(&tty_cdev, MKDEV(TTYAUX_MAJOR, 0), 1) || register_chrdev_region(MKDEV(TTYAUX_MAJOR, 0), 1, "/dev/tty") < 0) diff -ruPN a/include/linux/tty_buffer.h b/include/linux/tty_buffer.h --- a/include/linux/tty_buffer.h 2025-01-09 13:28:51.000000000 +0100 +++ b/include/linux/tty_buffer.h 2025-01-10 13:23:26.000000000 +0100 @@ -3,9 +3,9 @@ #define _LINUX_TTY_BUFFER_H #include +#include #include #include -#include struct tty_buffer { union { @@ -36,7 +36,7 @@ struct tty_bufhead { struct tty_buffer *head; /* Queue head */ - struct work_struct work; + struct kthread_work work; struct mutex lock; atomic_t priority; struct tty_buffer sentinel; diff -ruPN a/include/linux/tty_port.h b/include/linux/tty_port.h --- a/include/linux/tty_port.h 2025-01-09 13:28:51.000000000 +0100 +++ b/include/linux/tty_port.h 2025-01-10 14:43:23.909345579 +0100 @@ -61,7 +61,8 @@ wait_queue_head_t delta_msr_wait; /* Modem status change */ unsigned long flags; /* User TTY flags ASYNC_ */ unsigned long iflags; /* Internal flags TTY_PORT_ */ - unsigned char console:1; /* port is a console */ + unsigned char console:1, /* port is a console */ + low_latency:1; /* optional: tune for latency */ struct mutex mutex; /* Locking */ struct mutex buf_mutex; /* Buffer alloc lock */ unsigned char *xmit_buf; /* Optional buffer */