18.5. proc and sysfs Handling of TTY Devices
The tty core provides a very easy
way for any tty driver to maintain a
file in the /proc/tty/driver directory. If the
driver defines the read_proc or
write_proc functions, this file is created. Then, any read
or write call on this file is sent to the driver. The formats of
these functions are just like the standard /proc
file-handling functions. As an example, here is a simple implementation of the
read_proc tty callback that merely prints out
the number of the currently registered ports: static int tiny_read_proc(char *page, char **start, off_t off, int count,
int *eof, void *data)
{
struct tiny_serial *tiny;
off_t begin = 0;
int length = 0;
int i;
length += sprintf(page, "tinyserinfo:1.0 driver:%s\n", DRIVER_VERSION);
for (i = 0; i < TINY_TTY_MINORS && length < PAGE_SIZE; ++i) {
tiny = tiny_table[i];
if (tiny = = NULL)
continue;
length += sprintf(page+length, "%d\n", i);
if ((length + begin) > (off + count))
goto done;
if ((length + begin) < off) {
begin += length;
length = 0;
}
}
*eof = 1;
done:
if (off >= (length + begin))
return 0;
*start = page + (off-begin);
return (count < begin+length-off) ? count : begin + length-off;
}The tty core handles all of the sysfs directory and device creation
when the tty driver is registered, or when the individual tty devices
are created, depending on the TTY_DRIVER_NO_DEVFS
flag in the struct tty_driver.
The individual directory always contains the dev
file, which allows user-space tools to determine the major and minor
number assigned to the device. It also contains a
device and driver symlink,
if a pointer to a valid struct
device is passed in the call to
tty_register_device. Other than these three
files, it is not possible for individual tty drivers to create new
sysfs files in this location. This will probably change in future
kernel releases. |