[PATCH] TTY layer buffering revamp

The API and code have been through various bits of initial review by
serial driver people but they definitely need to live somewhere for a
while so the unconverted drivers can get knocked into shape, existing
drivers that have been updated can be better tuned and bugs whacked out.

This replaces the tty flip buffers with kmalloc objects in rings. In the
normal situation for an IRQ driven serial port at typical speeds the
behaviour is pretty much the same, two buffers end up allocated and the
kernel cycles between them as before.

When there are delays or at high speed we now behave far better as the
buffer pool can grow a bit rather than lose characters. This also means
that we can operate at higher speeds reliably.

For drivers that receive characters in blocks (DMA based, USB and
especially virtualisation) the layer allows a lot of driver specific
code that works around the tty layer with private secondary queues to be
removed. The IBM folks need this sort of layer, the smart serial port
people do, the virtualisers do (because a virtualised tty typically
operates at infinite speed rather than emulating 9600 baud).

Finally many drivers had invalid and unsafe attempts to avoid buffer
overflows by directly invoking tty methods extracted out of the innards
of work queue structs. These are no longer needed and all go away. That
fixes various random hangs with serial ports on overflow.

The other change in here is to optimise the receive_room path that is
used by some callers. It turns out that only one ldisc uses receive room
except asa constant and it updates it far far less than the value is
read. We thus make it a variable not a function call.

I expect the code to contain bugs due to the size alone but I'll be
watching and squashing them and feeding out new patches as it goes.

Because the buffers now dynamically expand you should only run out of
buffering when the kernel runs out of memory for real.  That means a lot of
the horrible hacks high performance drivers used to do just aren't needed any
more.

Description:

tty_insert_flip_char is an old API and continues to work as before, as does
tty_flip_buffer_push() [this is why many drivers dont need modification].  It
does now also return the number of chars inserted

There are also

tty_buffer_request_room(tty, len)

which asks for a buffer block of the length requested and returns the space
found.  This improves efficiency with hardware that knows how much to
transfer.

and tty_insert_flip_string_flags(tty, str, flags, len)

to insert a string of characters and flags

For a smart interface the usual code is

    len = tty_request_buffer_room(tty, amount_hardware_says);
    tty_insert_flip_string(tty, buffer_from_card, len);

More description!

At the moment tty buffers are attached directly to the tty.  This is causing a
lot of the problems related to tty layer locking, also problems at high speed
and also with bursty data (such as occurs in virtualised environments)

I'm working on ripping out the flip buffers and replacing them with a pool of
dynamically allocated buffers.  This allows both for old style "byte I/O"
devices and also helps virtualisation and smart devices where large blocks of
data suddenely materialise and need storing.

So far so good.  Lots of drivers reference tty->flip.*.  Several of them also
call directly and unsafely into function pointers it provides.  This will all
break.  Most drivers can use tty_insert_flip_char which can be kept as an API
but others need more.

At the moment I've added the following interfaces, if people think more will
be needed now is a good time to say

 int tty_buffer_request_room(tty, size)

Try and ensure at least size bytes are available, returns actual room (may be
zero).  At the moment it just uses the flipbuf space but that will change.
Repeated calls without characters being added are not cumulative.  (ie if you
call it with 1, 1, 1, and then 4 you'll have four characters of space.  The
other functions will also try and grow buffers in future but this will be a
more efficient way when you know block sizes.

 int tty_insert_flip_char(tty, ch, flag)

As before insert a character if there is room.  Now returns 1 for success, 0
for failure.

 int tty_insert_flip_string(tty, str, len)

Insert a block of non error characters.  Returns the number inserted.

 int tty_prepare_flip_string(tty, strptr, len)

Adjust the buffer to allow len characters to be added.  Returns a buffer
pointer in strptr and the length available.  This allows for hardware that
needs to use functions like insl or mencpy_fromio.

Signed-off-by: Alan Cox <alan@redhat.com>
Cc: Paul Fulghum <paulkf@microgate.com>
Signed-off-by: Hirokazu Takata <takata@linux-m32r.org>
Signed-off-by: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: John Hawkes <hawkes@sgi.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

4 files changed, 31 insertions, 25 deletions

diff --git a/include/linux/kbd_kern.h b/include/linux/kbd_kern.h
index 7428198..45f625d 100644
--- a/include/linux/kbd_kern.h
+++ b/include/linux/kbd_kern.h
@@ -151,7 +151,7 @@ extern unsigned int keymap_count;
 
 static inline void con_schedule_flip(struct tty_struct *t)
 {
-	schedule_work(&t->flip.work);
+	schedule_work(&t->buf.work);
 }
 
 #endif
diff --git a/include/linux/tty.h b/include/linux/tty.h
index 5744970..3787102 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -51,16 +51,22 @@
  */
 #define TTY_FLIPBUF_SIZE 512
 
-struct tty_flip_buffer {
+struct tty_buffer {
+	struct tty_buffer *next;
+	char *char_buf_ptr;
+	unsigned char *flag_buf_ptr;
+	int used;
+	int size;
+	/* Data points here */
+	unsigned long data[0];
+};
+
+struct tty_bufhead {
 	struct work_struct		work;
 	struct semaphore pty_sem;
-	char		*char_buf_ptr;
-	unsigned char	*flag_buf_ptr;
-	int		count;
-	int		buf_num;
-	unsigned char	char_buf[2*TTY_FLIPBUF_SIZE];
-	char		flag_buf[2*TTY_FLIPBUF_SIZE];
-	unsigned char	slop[4]; /* N.B. bug overwrites buffer by 1 */
+	struct tty_buffer *head;	/* Queue head */
+	struct tty_buffer *tail;	/* Active buffer */
+	struct tty_buffer *free;	/* Free queue head */
 };
 /*
  * The pty uses char_buf and flag_buf as a contiguous buffer
@@ -186,10 +192,11 @@ struct tty_struct {
 	unsigned char stopped:1, hw_stopped:1, flow_stopped:1, packet:1;
 	unsigned char low_latency:1, warned:1;
 	unsigned char ctrl_status;
+	unsigned int receive_room;	/* Bytes free for queue */
 
 	struct tty_struct *link;
 	struct fasync_struct *fasync;
-	struct tty_flip_buffer flip;
+	struct tty_bufhead buf;
 	int max_flip_cnt;
 	int alt_speed;		/* For magic substitution of 38400 bps */
 	wait_queue_head_t write_wait;
diff --git a/include/linux/tty_flip.h b/include/linux/tty_flip.h
index abe9bfc..be1400e 100644
--- a/include/linux/tty_flip.h
+++ b/include/linux/tty_flip.h
@@ -1,25 +1,33 @@
 #ifndef _LINUX_TTY_FLIP_H
 #define _LINUX_TTY_FLIP_H
 
+extern int tty_buffer_request_room(struct tty_struct *tty, size_t size);
+extern int tty_insert_flip_string(struct tty_struct *tty, unsigned char *chars, size_t size);
+extern int tty_insert_flip_string_flags(struct tty_struct *tty, unsigned char *chars, char *flags, size_t size);
+extern int tty_prepare_flip_string(struct tty_struct *tty, unsigned char **chars, size_t size);
+extern int tty_prepare_flip_string_flags(struct tty_struct *tty, unsigned char **chars, char **flags, size_t size);
+
 #ifdef INCLUDE_INLINE_FUNCS
 #define _INLINE_ extern
 #else
 #define _INLINE_ static __inline__
 #endif
 
-_INLINE_ void tty_insert_flip_char(struct tty_struct *tty,
+_INLINE_ int tty_insert_flip_char(struct tty_struct *tty,
 				   unsigned char ch, char flag)
 {
-	if (tty->flip.count < TTY_FLIPBUF_SIZE) {
-		tty->flip.count++;
-		*tty->flip.flag_buf_ptr++ = flag;
-		*tty->flip.char_buf_ptr++ = ch;
+	struct tty_buffer *tb = tty->buf.tail;
+	if (tb && tb->used < tb->size) {
+		tb->flag_buf_ptr[tb->used] = flag;
+		tb->char_buf_ptr[tb->used++] = ch;
+		return 1;
 	}
+	return tty_insert_flip_string_flags(tty, &ch, &flag, 1);
 }
 
 _INLINE_ void tty_schedule_flip(struct tty_struct *tty)
 {
-	schedule_delayed_work(&tty->flip.work, 1);
+	schedule_delayed_work(&tty->buf.work, 1);
 }
 
 #undef _INLINE_
diff --git a/include/linux/tty_ldisc.h b/include/linux/tty_ldisc.h
index 6066afd..83c6e6c 100644
--- a/include/linux/tty_ldisc.h
+++ b/include/linux/tty_ldisc.h
@@ -81,14 +81,6 @@
  * 	pointer of flag bytes which indicate whether a character was
  * 	received with a parity error, etc.
  * 
- * int	(*receive_room)(struct tty_struct *);
- *
- * 	This function is called by the low-level tty driver to
- * 	determine how many characters the line discpline can accept.
- * 	The low-level driver must not send more characters than was
- * 	indicated by receive_room, or the line discpline may drop
- * 	those characters.
- * 
  * void	(*write_wakeup)(struct tty_struct *);
  *
  * 	This function is called by the low-level tty driver to signal
@@ -136,7 +128,6 @@ struct tty_ldisc {
 	 */
 	void	(*receive_buf)(struct tty_struct *, const unsigned char *cp,
 			       char *fp, int count);
-	int	(*receive_room)(struct tty_struct *);
 	void	(*write_wakeup)(struct tty_struct *);
 
 	struct  module *owner;