tty: ldisc: document that ldops are optional

There is no need to provide any hook in struct tty_ldisc_ops. Document
that and write down that read/write return EIO in that case. The rest is
simply ignored.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230810091510.13006-3-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/tty_ldisc.h

@@ -71,7 +71,7 @@ int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass,
  *	call to @receive_buf(). Returning an error will prevent the ldisc from
  *	being attached.
  *
- *	Can sleep.
+ *	Optional. Can sleep.
  *
  * @close: [TTY] ``void ()(struct tty_struct *tty)``
  *
@@ -80,7 +80,7 @@ int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass,
  *	changed to use a new line discipline. At the point of execution no
  *	further users will enter the ldisc code for this tty.
  *
- *	Can sleep.
+ *	Optional. Can sleep.
  *
  * @flush_buffer: [TTY] ``void ()(struct tty_struct *tty)``
  *
@@ -88,6 +88,8 @@ int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass,
  *	input characters it may have queued to be delivered to the user mode
  *	process. It may be called at any point between open and close.
  *
+ *	Optional.
+ *
  * @read: [TTY] ``ssize_t ()(struct tty_struct *tty, struct file *file,
  *		unsigned char *buf, size_t nr)``
  *
@@ -97,7 +99,7 @@ int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass,
  *	an %EIO error. Multiple read calls may occur in parallel and the ldisc
  *	must deal with serialization issues.
  *
- *	Can sleep.
+ *	Optional: %EIO unless provided. Can sleep.
  *
  * @write: [TTY] ``ssize_t ()(struct tty_struct *tty, struct file *file,
  *		const unsigned char *buf, size_t nr)``
@@ -108,7 +110,7 @@ int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass,
  *	characters first. If this function is not defined, the user will
  *	receive an %EIO error.
  *
- *	Can sleep.
+ *	Optional: %EIO unless provided. Can sleep.
  *
  * @ioctl: [TTY] ``int ()(struct tty_struct *tty, unsigned int cmd,
  *		unsigned long arg)``
@@ -120,6 +122,8 @@ int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass,
  *	discpline. So a low-level driver can "grab" an ioctl request before
  *	the line discpline has a chance to see it.
  *
+ *	Optional.
+ *
  * @compat_ioctl: [TTY] ``int ()(struct tty_struct *tty, unsigned int cmd,
  *		unsigned long arg)``
  *
@@ -130,11 +134,15 @@ int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass,
  *	a pointer to wordsize-sensitive structure belongs here, but most of
  *	ldiscs will happily leave it %NULL.
  *
+ *	Optional.
+ *
  * @set_termios: [TTY] ``void ()(struct tty_struct *tty, const struct ktermios *old)``
  *
  *	This function notifies the line discpline that a change has been made
  *	to the termios structure.
  *
+ *	Optional.
+ *
  * @poll: [TTY] ``int ()(struct tty_struct *tty, struct file *file,
  *		  struct poll_table_struct *wait)``
  *
@@ -142,6 +150,8 @@ int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass,
  *	device. It is solely the responsibility of the line discipline to
  *	handle poll requests.
  *
+ *	Optional.
+ *
  * @hangup: [TTY] ``void ()(struct tty_struct *tty)``
  *
  *	Called on a hangup. Tells the discipline that it should cease I/O to
@@ -149,7 +159,7 @@ int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass,
  *	but should wait until any pending driver I/O is completed. No further
  *	calls into the ldisc code will occur.
  *
- *	Can sleep.
+ *	Optional. Can sleep.
  *
  * @receive_buf: [DRV] ``void ()(struct tty_struct *tty,
  *		       const unsigned char *cp, const char *fp, int count)``
@@ -161,6 +171,8 @@ int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass,
  *	character was received with a parity error, etc. @fp may be %NULL to
  *	indicate all data received is %TTY_NORMAL.
  *
+ *	Optional.
+ *
  * @write_wakeup: [DRV] ``void ()(struct tty_struct *tty)``
  *
  *	This function is called by the low-level tty driver to signal that line
@@ -170,11 +182,15 @@ int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass,
  *	send, please arise a tasklet or workqueue to do the real data transfer.
  *	Do not send data in this hook, it may lead to a deadlock.
  *
+ *	Optional.
+ *
  * @dcd_change: [DRV] ``void ()(struct tty_struct *tty, bool active)``
  *
  *	Tells the discipline that the DCD pin has changed its status. Used
  *	exclusively by the %N_PPS (Pulse-Per-Second) line discipline.
  *
+ *	Optional.
+ *
  * @receive_buf2: [DRV] ``int ()(struct tty_struct *tty,
  *			const unsigned char *cp, const char *fp, int count)``
  *
@@ -186,6 +202,8 @@ int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass,
  *	indicate all data received is %TTY_NORMAL. If assigned, prefer this
  *	function for automatic flow control.
  *
+ *	Optional.
+ *
  * @lookahead_buf: [DRV] ``void ()(struct tty_struct *tty,
  *			const unsigned char *cp, const char *fp, int count)``
  *
@@ -198,6 +216,8 @@ int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass,
  *	same characters (e.g. by skipping the actions for high-priority
  *	characters already handled by ->lookahead_buf()).
  *
+ *	Optional.
+ *
  * @owner: module containting this ldisc (for reference counting)
  *
  * This structure defines the interface between the tty line discipline