Integrate the new MPSAFE TTY layer to the FreeBSD operating system.

The last half year I've been working on a replacement TTY layer for the
FreeBSD kernel. The new TTY layer was designed to improve the following:

- Improved driver model:

  The old TTY layer has a driver model that is not abstract enough to
  make it friendly to use. A good example is the output path, where the
  device drivers directly access the output buffers. This means that an
  in-kernel PPP implementation must always convert network buffers into
  TTY buffers.

  If a PPP implementation would be built on top of the new TTY layer
  (still needs a hooks layer, though), it would allow the PPP
  implementation to directly hand the data to the TTY driver.

- Improved hotplugging:

  With the old TTY layer, it isn't entirely safe to destroy TTY's from
  the system. This implementation has a two-step destructing design,
  where the driver first abandons the TTY. After all threads have left
  the TTY, the TTY layer calls a routine in the driver, which can be
  used to free resources (unit numbers, etc).

  The pts(4) driver also implements this feature, which means
  posix_openpt() will now return PTY's that are created on the fly.

- Improved performance:

  One of the major improvements is the per-TTY mutex, which is expected
  to improve scalability when compared to the old Giant locking.
  Another change is the unbuffered copying to userspace, which is both
  used on TTY device nodes and PTY masters.

Upgrading should be quite straightforward. Unlike previous versions,
existing kernel configuration files do not need to be changed, except
when they reference device drivers that are listed in UPDATING.

Obtained from:		//depot/projects/mpsafetty/...
Approved by:		philip (ex-mentor)
Discussed:		on the lists, at BSDCan, at the DevSummit
Sponsored by:		Snow B.V., the Netherlands
dcons(4) fixed by:	kan

1 files changed, 19 insertions, 0 deletions

diff --git a/UPDATING b/UPDATING
index 81eaf5c0de64..d0ed6acb9a7c 100644
--- a/UPDATING
+++ b/UPDATING
@@ -22,6 +22,25 @@ NOTE TO PEOPLE WHO THINK THAT FreeBSD 8.x IS SLOW:
 	to maximize performance.  (To disable malloc debugging, run
 	ln -s aj /etc/malloc.conf.)
 
+20080820:
+	The TTY subsystem of the kernel has been replaced by a new
+	implementation, which provides better scalability and an
+	improved driver model. Most common drivers have been migrated to
+	the new TTY subsystem, while others have not. The following
+	drivers have not yet been ported to the new TTY layer:
+
+	PCI/ISA:
+		cy, digi, rc, rp, si, sio
+
+	USB:
+		ubser, ucycom, ufoma
+
+	Line disciplines:
+		ng_h4, ng_tty, ppp, sl, snp
+
+	Adding these drivers to your kernel configuration file shall
+	cause compilation to fail.
+
 20080713:
 	The sio(4) driver has been removed from the i386 and amd64
 	kernel configuration files. This means uart(4) is now the