TTY Line Discipline Issue with 3.10.y

[mholling]

I have been running a line discipline for the DNT900 radio module on Raspberry Pi using the 3.6 kernel under Arch Linux. It works well. When I update the kernel to 3.10 the line discipline stops working.

I have pared down the problem to the receive_buf function for the line discipline, which receives incoming characters from the serial port. The function gets called on the 3.6 kernel, but not the 3.10 kernel.

Reproduce the problem using the following module code:

#include <linux/module.h>
#include <linux/tty.h>
#include <linux/tty_ldisc.h>
#include <linux/printk.h>

#define N_TEST 29

static void test_receive_buf(struct tty_struct *tty, \
    const unsigned char *cp, char *fp, int count)
{
    pr_info("test_receive_buf called\n");
}

static struct tty_ldisc_ops test_ops = {
    .magic        = TTY_LDISC_MAGIC,
    .owner        = THIS_MODULE,
    .num          = N_TEST,
    .name         = "test",
    .receive_buf  = test_receive_buf,
};

int __init test_init(void)
{
    return tty_register_ldisc(N_TEST, &test_ops);
}

void __exit test_exit(void)
{
    tty_unregister_ldisc(N_TEST);
}

module_init(test_init);
module_exit(test_exit);

MODULE_LICENSE("GPL");

Compile and insert the module, then attach the line discipline (number 29) to the serial port:

insmod test.ko
ldattach -8n1 -s 115200 29 /dev/ttyAMA0

Under kernel 3.6, the test_receive_buf function is called and the log entry written when ttyAMA0 receives characters. Under kernel 3.10, the function is not called.

I have no idea if this is Pi-specific or an upstream kernel issue, or something else again!

-- Matthew

[popcornmix]

I have no idea, but when running on 3.6 kernel does lsmod show any other modules loaded?
Perhaps there is something required missing from 3.10 kernel.

[mholling]

The only difference I am seeing in modules is spidev, which is loaded on 3.6 but not on 3.10.

[mholling]

I solved this myself and will close the issue. The problem did not lie with the kernel. (Well except maybe its sparse documentation.)

For reference, it seems that line disciplines are meant to set (and optionally manage) the tty's receive_room value. Somehow in kernel 3.6 a default value was being set, whereas in 3.10 it defaults to zero, and thus the line discipline is not sent any data. Solved by (at minimum) setting the value in the line discipline's open operation:

...

static int test_open(struct tty_struct *tty)
{
    tty->receive_room = 4096;
    return 0;
}

static struct tty_ldisc_ops test_ops = {
    .magic        = TTY_LDISC_MAGIC,
    .owner        = THIS_MODULE,
    .num          = N_TEST,
    .name         = "test",
    .receive_buf  = test_receive_buf,
    .open         = test_open,
};

...

Code for the default N_TTY line discipline demonstrates management of the tty->receive_room value.

[popcornmix]

Good to hear it's fixed (as I had no ideas).