On 03/09/14 00:02, Thomas Gleixner wrote:

On Tue, 2 Sep 2014, Daniel Thompson wrote:

quoted

This patchset makes it possible to use kgdb's NMI infrastructure on ARM
platforms.

The patches are seperated into three distinct groups:

1. arm specific changes; these provide multi-platform support for FIQ
   (including raising an IPI using FIQ to ensure effective SMP support)
   and extend ARM KGDB support to use the features provided.

2. irqchip changes; updates to the gic and vic drivers to provide
   support for routing certain interrupt sources to FIQ.

3. serial changes; driver support to allow the UART interrupt to be
   routed to FIQ. The already mainlined kgdb NMI infrastructure (mostly
   found in drivers/tty/serial/kgdb_nmi.c) will re-route the kgdb
   console UART's interrupt signal from IRQ to FIQ. Naturally the UART
   will no longer function normally and will instead be managed by kgdb
   using the polled I/O functions. Any character delivered to the UART
   causes the kgdb handler function to be called.

To be honest, what you are doing is just ass backwards.

The use case you are looking for is the most irrelevant of all. Just
because KGDB is on some managerial "must have items" checklist does
not make it useful.

The FIQ based interactive debugger use case is fairly common on Android,
especially for Nexus devices (they have an out-of-tree debugger similar
to kdb for this).

I think it finds favour there because during the development phases
where the console is unplugged to allow developers to go walkabout live
with a prototype phone. The interactive debugger is used for
post-morteming when something breaks. At this stage of development are
reluctant to expose/consume hardware resources (JTAG pins, RAM, FLASH)
for JTAG or kexec/kdump post-mortems.

The only relevant use cases of FIQs are the same as those of NMIs on
x86:

  - Watchdog to detect stuck cpus and issue stack traces

Russell put together a quick 'n dirty version of the NMI stack trace
code based on a subset of my patchset. Based on his feedback, later
revisions of my patchset are structured to simplify adding this code.


Daniel.

  - Performace monitoring

KGDB falls into place once you solved the above.

That said for the general approach, I'll have a look at the irq
related patches in a minute.

Thanks,

	tglx