[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>

1 files changed, 30 insertions, 43 deletions

diff --git a/drivers/char/moxa.c b/drivers/char/moxa.c
index 46a3a8ccd65..5e3ef552219 100644
--- a/drivers/char/moxa.c
+++ b/drivers/char/moxa.c
@@ -269,7 +269,7 @@ static int MoxaPortDCDChange(int);
 static int MoxaPortDCDON(int);
 static void MoxaPortFlushData(int, int);
 static int MoxaPortWriteData(int, unsigned char *, int);
-static int MoxaPortReadData(int, unsigned char *, int);
+static int MoxaPortReadData(int, struct tty_struct *tty);
 static int MoxaPortTxQueue(int);
 static int MoxaPortRxQueue(int);
 static int MoxaPortTxFree(int);
@@ -301,6 +301,8 @@ static struct tty_operations moxa_ops = {
         .tiocmset = moxa_tiocmset,
 };
 
+static spinlock_t moxa_lock = SPIN_LOCK_UNLOCKED;
+
 #ifdef CONFIG_PCI
 static int moxa_get_PCI_conf(struct pci_dev *p, int board_type, moxa_board_conf * board)
 {
@@ -645,10 +647,10 @@ static int moxa_write(struct tty_struct *tty,
         if (ch == NULL)
                 return (0);
         port = ch->port;
-        save_flags(flags);
-        cli();
+
+        spin_lock_irqsave(&moxa_lock, flags);
         len = MoxaPortWriteData(port, (unsigned char *) buf, count);
-        restore_flags(flags);
+        spin_unlock_irqrestore(&moxa_lock, flags);
 
         /*********************************************
         if ( !(ch->statusflags & LOWWAIT) &&
@@ -723,11 +725,10 @@ static void moxa_put_char(struct tty_struct *tty, unsigned char c)
         if (ch == NULL)
                 return;
         port = ch->port;
-        save_flags(flags);
-        cli();
+        spin_lock_irqsave(&moxa_lock, flags);
         moxaXmitBuff[0] = c;
         MoxaPortWriteData(port, moxaXmitBuff, 1);
-        restore_flags(flags);
+        spin_unlock_irqrestore(&moxa_lock, flags);
         /************************************************
         if ( !(ch->statusflags & LOWWAIT) && (MoxaPortTxFree(port) <= 100) )
         *************************************************/
@@ -1030,12 +1031,12 @@ static int block_till_ready(struct tty_struct *tty, struct file *filp,
         printk("block_til_ready before block: ttys%d, count = %d\n",
                ch->line, ch->count);
 #endif
-        save_flags(flags);
-        cli();
+        spin_lock_irqsave(&moxa_lock, flags);
         if (!tty_hung_up_p(filp))
                 ch->count--;
-        restore_flags(flags);
         ch->blocked_open++;
+        spin_unlock_irqrestore(&moxa_lock, flags);
+
         while (1) {
                 set_current_state(TASK_INTERRUPTIBLE);
                 if (tty_hung_up_p(filp) ||
@@ -1062,17 +1063,21 @@ static int block_till_ready(struct tty_struct *tty, struct file *filp,
         }
         set_current_state(TASK_RUNNING);
         remove_wait_queue(&ch->open_wait, &wait);
+
+        spin_lock_irqsave(&moxa_lock, flags);
         if (!tty_hung_up_p(filp))
                 ch->count++;
         ch->blocked_open--;
+        spin_unlock_irqrestore(&moxa_lock, flags);
 #ifdef SERIAL_DEBUG_OPEN
         printk("block_til_ready after blocking: ttys%d, count = %d\n",
                ch->line, ch->count);
 #endif
         if (retval)
                 return (retval);
+        /* FIXME: review to see if we need to use set_bit on these */
         ch->asyncflags |= ASYNC_NORMAL_ACTIVE;
-        return (0);
+        return 0;
 }
 
 static void setup_empty_event(struct tty_struct *tty)
@@ -1080,15 +1085,14 @@ static void setup_empty_event(struct tty_struct *tty)
         struct moxa_str *ch = tty->driver_data;
         unsigned long flags;
 
-        save_flags(flags);
-        cli();
+        spin_lock_irqsave(&moxa_lock, flags);
         ch->statusflags |= EMPTYWAIT;
         moxaEmptyTimer_on[ch->port] = 0;
         del_timer(&moxaEmptyTimer[ch->port]);
         moxaEmptyTimer[ch->port].expires = jiffies + HZ;
         moxaEmptyTimer_on[ch->port] = 1;
         add_timer(&moxaEmptyTimer[ch->port]);
-        restore_flags(flags);
+        spin_unlock_irqrestore(&moxa_lock, flags);
 }
 
 static void check_xmit_empty(unsigned long data)
@@ -1135,8 +1139,6 @@ static void receive_data(struct moxa_str *ch)
 {
         struct tty_struct *tp;
         struct termios *ts;
-        int i, count, rc, space;
-        unsigned char *charptr, *flagptr;
         unsigned long flags;
 
         ts = NULL;
@@ -1150,24 +1152,10 @@ static void receive_data(struct moxa_str *ch)
                 MoxaPortFlushData(ch->port, 0);
                 return;
         }
-        space = TTY_FLIPBUF_SIZE - tp->flip.count;
-        if (space <= 0)
-                return;
-        charptr = tp->flip.char_buf_ptr;
-        flagptr = tp->flip.flag_buf_ptr;
-        rc = tp->flip.count;
-        save_flags(flags);
-        cli();
-        count = MoxaPortReadData(ch->port, charptr, space);
-        restore_flags(flags);
-        for (i = 0; i < count; i++)
-                *flagptr++ = 0;
-        charptr += count;
-        rc += count;
-        tp->flip.count = rc;
-        tp->flip.char_buf_ptr = charptr;
-        tp->flip.flag_buf_ptr = flagptr;
-        tty_schedule_flip(ch->tty);
+        spin_lock_irqsave(&moxa_lock, flags);
+        MoxaPortReadData(ch->port, tp);
+        spin_unlock_irqrestore(&moxa_lock, flags);
+        tty_schedule_flip(tp);
 }
 
 #define Magic_code      0x404
@@ -1774,7 +1762,7 @@ int MoxaPortsOfCard(int cardno)
  *      14. MoxaPortDCDON(int port);                                         *
  *      15. MoxaPortFlushData(int port, int mode);                           *
  *      16. MoxaPortWriteData(int port, unsigned char * buffer, int length); *
- *      17. MoxaPortReadData(int port, unsigned char * buffer, int length);  *
+ *      17. MoxaPortReadData(int port, struct tty_struct *tty);              *
  *      18. MoxaPortTxBufSize(int port);                                     *
  *      19. MoxaPortRxBufSize(int port);                                     *
  *      20. MoxaPortTxQueue(int port);                                       *
@@ -2003,10 +1991,9 @@ int MoxaPortsOfCard(int cardno)
  *
  *      Function 21:    Read data.
  *      Syntax:
- *      int  MoxaPortReadData(int port, unsigned char * buffer, int length);
+ *      int  MoxaPortReadData(int port, struct tty_struct *tty);
  *           int port           : port number (0 - 127)
- *           unsigned char * buffer     : pointer to read data buffer.
- *           int length         : read data buffer length
+ *           struct tty_struct *tty : tty for data
  *
  *           return:    0 - length      : real read data length
  *
@@ -2504,7 +2491,7 @@ int MoxaPortWriteData(int port, unsigned char * buffer, int len)
         return (total);
 }
 
-int MoxaPortReadData(int port, unsigned char * buffer, int space)
+int MoxaPortReadData(int port, struct tty_struct *tty)
 {
         register ushort head, pageofs;
         int i, count, cnt, len, total, remain;
@@ -2522,9 +2509,9 @@ int MoxaPortReadData(int port, unsigned char * buffer, int space)
         count = (tail >= head) ? (tail - head)
             : (tail - head + rx_mask + 1);
         if (count == 0)
-                return (0);
+                return 0;
 
-        total = (space > count) ? count : space;
+        total = count;
         remain = count - total;
         moxaLog.rxcnt[port] += total;
         count = total;
@@ -2539,7 +2526,7 @@ int MoxaPortReadData(int port, unsigned char * buffer, int space)
                         len = (count > len) ? len : count;
                         ofs = baseAddr + DynPage_addr + bufhead + head;
                         for (i = 0; i < len; i++)
-                                *buffer++ = readb(ofs + i);
+                                tty_insert_flip_char(tty, readb(ofs + i), TTY_NORMAL);
                         head = (head + len) & rx_mask;
                         count -= len;
                 }
@@ -2556,7 +2543,7 @@ int MoxaPortReadData(int port, unsigned char * buffer, int space)
                         writew(pageno, baseAddr + Control_reg);
                         ofs = baseAddr + DynPage_addr + pageofs;
                         for (i = 0; i < cnt; i++)
-                                *buffer++ = readb(ofs + i);
+                                tty_insert_flip_char(tty, readb(ofs + i), TTY_NORMAL);
                         if (count == 0) {
                                 writew((head + len) & rx_mask, ofsAddr + RXrptr);
                                 break;