mutexes or using any callouts when active.
Trying to lock a mutex when KDB is active or the scheduler is stopped
can result in infinite wait loops. The same goes for calling callout
related functions which in turn lock mutexes.
If the USB controller at which a USB keyboard is connected is idle
when KDB is entered, polling the USB keyboard via USB will always
succeed. Else polling may fail depending on which state the USB
subsystem and USB interrupt handler is in. This is unavoidable unless
KDB can wait for USB interrupt threads to complete before stalling the
CPU(s).
Tested by: Bruce Evans <bde@freebsd.org>
MFC after: 4 weeks
into the FreeBSD boot loader, typically for non-USB aware BIOSes, EFI systems
or embedded platforms. This is also useful for out of the system compilation
of the FreeBSD USB stack for various purposes. The USB kernel files can
now optionally include a global header file which should include all needed
definitions required to compile the FreeBSD USB stack. When the global USB
header file is included, no other USB header files will be included by
default.
Add new file containing the USB stack configuration for the
FreeBSD loader build.
Replace some __FBSDID()'s by /* $FreeBSD$ */ comments. Now all
USB files follow the same style.
Use cases:
- console in loader via USB
- loading kernel via USB
Discussed with: Hiroki Sato, hrs @ EuroBSDCon
