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, 54 insertions, 42 deletions

diff --git a/share/man/man4/da.4 b/share/man/man4/da.4
index 835f4fd..c35b59f 100644
--- a/share/man/man4/da.4
+++ b/share/man/man4/da.4
@@ -25,7 +25,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd October 15, 1998
+.Dd September 2, 2003
 .Dt DA 4
 .Os
 .Sh NAME
@@ -196,6 +196,59 @@ The driver
 .Em will
 write the new disklabel to the disk.
 .El
+.Sh SYSCTL VARIABLES
+The following variables are available as both
+.Xr sysctl 8
+variables and
+.Xr loader 8
+tunables:
+.Bl -tag -width 12
+.It kern.cam.da.retry_count
+.Pp
+This variable determines how many times the
+.Nm
+driver will retry a READ or WRITE command.
+This does not affect the number of retries used during probe time or for
+the
+.Nm
+driver dump routine.
+This value currently defaults to 4.
+.It kern.cam.da.default_timeout
+.Pp
+This variable determines how long the
+.Nm
+driver will wait before timing out an outstanding command.
+The units for this value are seconds, and the default is currently 60
+seconds.
+.It kern.cam.da.%d.minimum_cmd_size
+.Pp
+This variable determines what the minimum READ/WRITE CDB size is for a
+given
+.Nm
+unit.
+(The %d above denotes the unit number of the
+.Nm
+driver instance, e.g. 1, 2, 4, 8, etc.)
+Valid minimum command size values are 6, 10, 12 and 16 bytes.
+The default is 6 bytes.
+.Pp
+The
+.Nm
+driver issues a CAM Path Inquiry CCB at probe time to determine whether the
+protocol the device in question speaks (e.g. ATAPI) typically doesn't allow
+6 byte commands.
+If it doesn't, the
+.Nm
+driver will default to using at least 10 byte CDBs.
+If a 6 byte READ or WRITE fails with an ILLEGAL REQUEST error, the
+.Nm
+driver will then increase the default CDB size for the device to 10 bytes and
+retry the command.
+CDB size is always
+chosen as the smallest READ/WRITE CDB that will satisfy the specified minimum
+command size, and the LBA and length of the READ or WRITE in question.
+(e.g., a write to an LBA larger than 2^32 will require a 16 byte CDB.)
+.El
 .Sh NOTES
 If a device becomes invalidated (media is removed, device becomes unresponsive)
 the disklabel and information held within the kernel about the device will
@@ -206,25 +259,9 @@ the last file descriptor referencing the old device is closed.
 During this period, all new open attempts will be rejected.
 .Sh FILES
 .Bl -tag -width /dev/rsdXXXXX -compact
-.It Pa /dev/rda Ns Ar u
-raw mode
-.Tn SCSI
-disk unit
-.Ar u ,
-accessed as an unpartitioned device
 .Sm off
 .It Pa /dev/da Ar u Pa s Ar n
 .Sm on
-block mode
-.Tn SCSI
-disk unit
-.Ar u ,
-slice
-.Ar n ,
-accessed as an unpartitioned device
-.Sm off
-.It Pa /dev/rda Ar u Pa s Ar n
-.Sm on
 raw mode
 .Tn SCSI
 disk unit
@@ -233,15 +270,6 @@ slice
 .Ar n ,
 accessed as an unpartitioned device
 .It Pa /dev/da Ns Ar u Ns Ar p
-block mode
-.Tn SCSI
-disk unit
-.Ar u ,
-first
-.Fx
-slice, partition
-.Ar p
-.It Pa /dev/rda Ns Ar u Ns Ar p
 raw mode
 .Tn SCSI
 disk unit
@@ -259,22 +287,6 @@ slice, partition
 .Ar p
 .Xc
 .Sm on
-block mode
-.Tn SCSI
-disk unit
-.Ar u ,
-.Ar n Ns th
-slice, partition
-.Ar p
-.Sm off
-.It Xo
-.Pa /dev/rda
-.Ar u
-.Pa s
-.Ar n
-.Ar p
-.Xc
-.Sm on
 raw mode
 .Tn SCSI
 disk unit