Move dynamic sysctl(8) variable creation for the cd(4) and da(4) drivers

out of cdregister() and daregister(), which are run from interrupt context.

The sysctl code does blocking mallocs (M_WAITOK), which causes problems
if malloc(9) actually needs to sleep.

The eventual fix for this issue will involve moving the CAM probe process
inside a kernel thread.  For now, though, I have fixed the issue by moving
dynamic sysctl variable creation for these two drivers to a task queue
running in a kernel thread.

The existing task queues (taskqueue_swi and taskqueue_swi_giant) run in
software interrupt handlers, which wouldn't fix the problem at hand.  So I
have created a new task queue, taskqueue_thread, that runs inside a kernel
thread.  (It also runs outside of Giant -- clients must explicitly acquire
and release Giant in their taskqueue functions.)

scsi_cd.c:	Remove sysctl variable creation code from cdregister(), and
		move it to a new function, cdsysctlinit().  Queue
		cdsysctlinit() to the taskqueue_thread taskqueue once we
		have fully registered the cd(4) driver instance.

scsi_da.c:	Remove sysctl variable creation code from daregister(), and
		move it to move it to a new function, dasysctlinit().
		Queue dasysctlinit() to the taskqueue_thread taskqueue once
		we have fully registered the da(4) instance.

taskqueue.h:	Declare the new taskqueue_thread taskqueue, update some
		comments.

subr_taskqueue.c:
		Create the new kernel thread taskqueue.  This taskqueue
		runs outside of Giant, so any functions queued to it would
		need to explicitly acquire/release Giant if they need it.

cd.4:		Update the cd(4) man page to talk about the minimum command
		size sysctl/loader tunable.  Also note that the changer
		variables are available as loader tunables as well.

da.4:		Update the da(4) man page to cover the retry_count,
		default_timeout and minimum_cmd_size sysctl variables/loader
		tunables.  Remove references to /dev/r???, they aren't used
		any longer.

cd.9:		Update the cd(9) man page to describe the CD_Q_10_BYTE_ONLY
		quirk.

taskqueue.9:	Update the taskqueue(9) man page to describe the new thread
		task queue, and the taskqueue_swi_giant queue.

MFC after:	3 days

1 files changed, 34 insertions, 3 deletions

diff --git a/share/man/man4/cd.4 b/share/man/man4/cd.4
index 579fcea..7c491f5 100644
--- a/share/man/man4/cd.4
+++ b/share/man/man4/cd.4
@@ -25,7 +25,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd October 10, 1998
+.Dd September 2, 2003
 .Dt CD 4
 .Os
 .Sh NAME
@@ -370,6 +370,37 @@ Some work is planned to support
 some of the more common `broken'
 .Tn CD-ROM
 drives; however, this is not yet under way.
+.Pp
+The
+.Nm
+driver attempts to automatically determine whether the drive it is talking
+to supports 6 byte or 10 byte MODE SENSE/MODE SELECT operations.  Many
+.Tn SCSI
+drives only support 6 byte commands, and
+.Tn ATAPI
+drives only support 10 byte commands.
+The
+.Nm
+driver first attempts to determine whether the protocol in use typically
+supports 6 byte commands by issuing a CAM Path Inquiry CCB.
+It will then default to 6 byte or 10 byte commands as appropriate.
+After that, the
+.Nm
+driver defaults to using 6 byte commands (assuming the protocol the drive
+speaks claims to support 6 byte commands), until one fails with a
+.Tn SCSI 
+ILLEGAL REQUEST error.  Then it tries the 10 byte version of the command to
+see if that works instead.  Users can change the default via per-drive
+sysctl variables and loader tunables.  The variable names are the same in
+both instances:
+.Pp
+.Va kern.cam.cd.%d.minimum_cmd_size
+.Pp
+Where 
+.Dq %d
+is the unit number of the drive in question.  Valid minimum command sizes
+are 6 and 10.  Any value above 6 will be rounded to 10, and any value below
+6 will be rounded to 6.
 .Sh CHANGER OPERATION
 This driver has built-in support for LUN-based CD changers.
 A LUN-based CD
@@ -399,7 +430,7 @@ If there is no outstanding I/O for
 another LUN, the driver will allow indefinite access to a given LUN.
 .Pp
 The minimum and maximum time quanta are configurable via kernel options and
-also via sysctl variables.
+also via sysctl and kernel tunable variables.
 The kernel options are:
 .Pp
 .Bl -item -compact
@@ -409,7 +440,7 @@ The kernel options are:
 .Cd "options ""CHANGER_MAX_BUSY_SECONDS=11"""
 .El
 .Pp
-The sysctl variables are:
+The sysctl/kernel tunable variables are:
 .Pp
 .Bl -item -compact
 .It