Make it possible for UARTs to trigger magic sysrq from an NMI. With the advent of pseudo NMIs on arm64 it became quite generic to request serial device interrupt as an NMI rather than IRQ. And having NMI driven serial RX will allow us to trigger magic sysrq as an NMI and hence drop into kernel debugger in NMI context. The major use-case is to add NMI debugging capabilities to the kernel in order to debug scenarios such as: - Primary CPU is stuck in deadlock with interrupts disabled and hence doesn't honor serial device interrupt. So having magic sysrq triggered as an NMI is helpful for debugging. - Always enabled NMI based magic sysrq irrespective of whether the serial TTY port is active or not. Currently there is an existing kgdb NMI serial driver which provides partial implementation in upstream to have a separate ttyNMI0 port but that remained in silos with the serial core/drivers which made it a bit odd to enable using serial device interrupt and hence remained unused. It seems to be clearly intended to avoid almost all custom NMI changes to the UART driver. But this patch-set allows the serial core/drivers to be NMI aware which in turn provides NMI debugging capabilities via magic sysrq and hence there is no specific reason to keep this special driver. So remove it instead. Approach: --------- The overall idea is to intercept serial RX characters in NMI context, if those are specific to magic sysrq then allow corresponding handler to run in NMI context. Otherwise, defer all other RX and TX operations onto IRQ work queue in order to run those in normal interrupt context. This approach is demonstrated using amba-pl011 driver. Patch-wise description: ----------------------- Patch #1 prepares magic sysrq handler to be NMI aware. Patch #2 adds NMI framework to serial core. Patch #3 and #4 demonstrates NMI aware uart port using amba-pl011 driver. Patch #5 removes kgdb NMI serial driver. Goal of this RFC: ----------------- My main reason for sharing this as an RFC is to help decide whether or not to continue with this approach. The next step for me would to port the work to a system with an 8250 UART. Usage: ------ This RFC has been developed on top of 5.8-rc3 and if anyone is interested to give this a try on QEMU, just enable following config options additional to arm64 defconfig: CONFIG_KGDB=y CONFIG_KGDB_KDB=y CONFIG_ARM64_PSEUDO_NMI=y Qemu command line to test: $ qemu-system-aarch64 -nographic -machine virt,gic-version=3 -cpu cortex-a57 \ -smp 2 -kernel arch/arm64/boot/Image -append 'console=ttyAMA0,38400 \ keep_bootcon root=/dev/vda2 irqchip.gicv3_pseudo_nmi=1 kgdboc=ttyAMA0' \ -initrd rootfs-arm64.cpio.gz NMI entry into kgdb via sysrq: - Ctrl a + b + g Reference: ---------- For more details about NMI/FIQ debugger, refer to this blog post [1]. [1] https://www.linaro.org/blog/debugging-arm-kernels-using-nmifiq/ I do look forward to your comments and feedback. Sumit Garg (5): tty/sysrq: Make sysrq handler NMI aware serial: core: Add framework to allow NMI aware serial drivers serial: amba-pl011: Re-order APIs definition serial: amba-pl011: Enable NMI aware uart port serial: Remove KGDB NMI serial driver drivers/tty/serial/Kconfig | 19 -- drivers/tty/serial/Makefile | 1 - drivers/tty/serial/amba-pl011.c | 232 +++++++++++++++++------- drivers/tty/serial/kgdb_nmi.c | 383 --------------------------------------- drivers/tty/serial/kgdboc.c | 8 - drivers/tty/serial/serial_core.c | 120 +++++++++++- drivers/tty/sysrq.c | 33 +++- include/linux/kgdb.h | 10 - include/linux/serial_core.h | 67 +++++++ include/linux/sysrq.h | 1 + kernel/debug/debug_core.c | 1 + 11 files changed, 386 insertions(+), 489 deletions(-) delete mode 100644 drivers/tty/serial/kgdb_nmi.c -- 2.7.4
