Hi Greg, Thanks for your comments. On Tue, 11 Aug 2020 at 19:27, Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> wrote: > > On Tue, Aug 11, 2020 at 07:20:26PM +0530, Sumit Garg wrote: > > On Tue, 21 Jul 2020 at 17:40, Sumit Garg <sumit.garg@xxxxxxxxxx> wrote: > > > > > > 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. > > > > > > > A gentle reminder to seek feedback on this series. > > It's the middle of the merge window, and I can't do anything. > > Also, I almost never review RFC patches as I have have way too many > patches that people think are "right" to review first... > Okay, I understand and I can definitely wait for your feedback. > I suggest you work to flesh this out first and submit something that you > feels works properly. > IIUC, in order to make this approach substantial I need to make it work with 8250 UART (major serial driver), correct? As currently it works properly for amba-pl011 driver. > good luck! > Thanks. -Sumit > greg k-h
