[PATCH v3 4/4] printk/nmi: Increase the size of NMI buffer and make it configurable

[Andrew Morton]

On Fri, 11 Dec 2015 23:21:13 +0000 Russell King - ARM Linux <linux at arm.linux.org.uk> wrote:

> On Fri, Dec 11, 2015 at 02:57:25PM -0800, Andrew Morton wrote:
> > This is a bit messy.  NEED_PRINTK_NMI is an added-on hack for one
> > particular arm variant.  From the changelog:
> > 
> >    "One exception is arm where the deferred printing is used for
> >     printing backtraces even without NMI.  For this purpose, we define
> >     NEED_PRINTK_NMI Kconfig flag.  The alternative printk_func is
> >     explicitly set when IPI_CPU_BACKTRACE is handled."
> > 
> > 
> > - why does arm needs deferred printing for backtraces?
> > 
> > - why is this specific to CONFIG_CPU_V7M?
> > 
> > - can this Kconfig logic be cleaned up a bit?
> 
> I think this comes purely from this attempt to apply another round of
> cleanups to the nmi backtrace work I did.
> 
> As I explained when I did that work, the vast majority of ARM platforms
> are unable to trigger anything like a NMI - the FIQ is something that's
> generally a property of the secure monitor, and is not accessible to
> Linux.  However, there are platforms where it is accessible.

OK, thanks.  So "not needed at present, might be needed in the future,
useful for out-of-tree debug code"?

> I'm personally happy with the existing code, and I've been wondering why
> there's this effort to apply further cleanups - to me, the changelogs
> don't seem to make that much sense, unless we want to start using
> printk() extensively in NMI functions - using the generic nmi backtrace
> code surely gets us something that works across all architectures...

Yes, I was scratching my head over that.  The patchset takes an nmi-safe
all-cpu-backtrace and generalises that into an nmi-safe printk.  That
*sounds* like a good thing to do but yes, some additional justification
would be helpful.  What real-world value does this patchset really
bring to real-world users?