[V3] powerpc/powernv: Add a kmsg_dumper that flushes console output on panic

[Michael Ellerman]

On Fri, 2015-27-11 at 06:23:07 UTC, Russell Currey wrote:
> On BMC machines, console output is controlled by the OPAL firmware and is
> only flushed when its pollers are called.  When the kernel is in a panic
> state, it no longer calls these pollers and thus console output does not
> completely flush, causing some output from the panic to be lost.
> 
> Output is only actually lost when the kernel is configured to not power off
> or reboot after panic (i.e. CONFIG_PANIC_TIMEOUT is set to 0) since OPAL
> flushes the console buffer as part of its power down routines.  Before this
> patch, however, only partial output would be printed during the timeout wait.
> 
> This patch adds a new kmsg_dumper which gets called at panic time to ensure
> panic output is not lost.  It accomplishes this by calling OPAL_CONSOLE_FLUSH
> in the OPAL API, and if that is not available, the pollers are called enough
> times to (hopefully) completely flush the buffer.
> 
> The flushing mechanism will only affect output printed at and before the
> kmsg_dump call in kernel/panic.c:panic().  As such, the "end Kernel panic"
> message may still be truncated as follows:
> 
> >Call Trace:
> >[c000000f1f603b00] [c0000000008e9458] dump_stack+0x90/0xbc (unreliable)
> >[c000000f1f603b30] [c0000000008e7e78] panic+0xf8/0x2c4
> >[c000000f1f603bc0] [c000000000be4860] mount_block_root+0x288/0x33c
> >[c000000f1f603c80] [c000000000be4d14] prepare_namespace+0x1f4/0x254
> >[c000000f1f603d00] [c000000000be43e8] kernel_init_freeable+0x318/0x350
> >[c000000f1f603dc0] [c00000000000bd74] kernel_init+0x24/0x130
> >[c000000f1f603e30] [c0000000000095b0] ret_from_kernel_thread+0x5c/0xac
> >---[ end Kernel panic - not
> 
> This functionality is implemented as a kmsg_dumper as it seems to be the
> most sensible way to introduce platform-specific functionality to the
> panic function.
> 
> Signed-off-by: Russell Currey <ruscur at russell.cc>
> Reviewed-by: Andrew Donnellan <andrew.donnellan at au1.ibm.com>

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/affddff69c55eb68969448f35f

cheers