Date: Sun, 17 Feb 2008 22:24:57 +0100 From: Ed Schouten <ed@fxq.nl> To: freebsd-arch@freebsd.org Subject: Device minor number uniqueness Message-ID: <20080217212457.GX1340@hoeg.nl>
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Hello,
This week I was looking at the ttycreate() function in tty.c. As you can
see, it generates unique numbers and passes them to make_dev() to make
sure each TTY device node has its own number.
The manual page has a rather brief description of the `minor' argument
of make_dev():
The cdev returned by make_dev() and make_dev_alias() has two fields,
si_drv1 and si_drv2, that are available to store state. Both fields a=
re
of type void *. These are designed to replace the minor argument to
make_dev().
I discovered you get a panic when you call make_dev() multiple times
with the same minor number, because newdev() always walks down the
device list to return an existing device which shares the same minor
number.
After digging into some more source code, it turns out a lot of drivers
use minor number to store device numbers and such, but there is no real
reason why we should enforce drivers to use unique minor numbers.
Because we cannot change this behaviour at once, I'm proposing the
following patch, which adds a new flag called D_MULTIMINOR. This flag
allows you to create multiple devices that share the same minor number.
This way there is no need for creating your own unrhdr to hold some kind
of free list.
--=20
Ed Schouten <ed@fxq.nl>
WWW: http://g-rave.nl/
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=3D=3D=3D=3D //depot/user/ed/mpsafetty/sys/kern/kern_conf.c#3 - /home/ed/p4=
/mpsafetty/sys/kern/kern_conf.c =3D=3D=3D=3D
@@ -458,10 +458,12 @@
=20
mtx_assert(&devmtx, MA_OWNED);
udev =3D y;
- LIST_FOREACH(si2, &csw->d_devs, si_list) {
- if (si2->si_drv0 =3D=3D udev) {
- dev_free_devlocked(si);
- return (si2);
+ if (!(csw->d_flags & D_MULTIMINOR)) {
+ LIST_FOREACH(si2, &csw->d_devs, si_list) {
+ if (si2->si_drv0 =3D=3D udev) {
+ dev_free_devlocked(si);
+ return (si2);
+ }
}
}
si->si_drv0 =3D udev;
=3D=3D=3D=3D //depot/user/ed/mpsafetty/sys/sys/conf.h#3 - /home/ed/p4/mpsaf=
etty/sys/sys/conf.h =3D=3D=3D=3D
@@ -171,6 +171,7 @@
#define D_MMAP_ANON 0x00100000 /* special treatment in vm_mmap.c */
#define D_PSEUDO 0x00200000 /* make_dev() can return NULL */
#define D_NEEDGIANT 0x00400000 /* driver want Giant */
+#define D_MULTIMINOR 0x00800000 /* don't track minor numbers */
=20
/*
* Version numbers.
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