linux-kernel - Re: [PATCH 11/13] android: persistent_trace: ftrace into persistent

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Date:	Wed, 07 Mar 2012 14:40:54 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Colin Cross <ccross@...roid.com>,
	Android Kernel Team <kernel-team@...roid.com>,
	tony.luck@...el.com
Subject: Re: [PATCH 11/13] android: persistent_trace: ftrace into
 persistent_ram

On Wed, 2012-03-07 at 14:10 -0800, Greg KH wrote:
> On Wed, Mar 07, 2012 at 01:58:21PM -0800, John Stultz wrote:
> > From: Colin Cross <ccross@...roid.com>
> > 
> > persistent_trace uses the ftrace infrastructure, but traces
> > into a persistent_ram buffer instead of the regular ftrace
> > ringbuffer.  After a reset or panic, the trace can be
> > decoded with cat /sys/kernel/debug/persistent_trace.
> 
> Has this patch been run by the tracing kernel developers?

Probably not. If you'd prefer, we can drop this one and come back to it
later.

> And what's the status on merging the persistent_ram stuff in with the
> in-kernel api for this type of thing?  That should then get rid of this
> separate tracing module, or at the least, let it be accepted directly by
> the tracing developers and not need to go through staging, right?

>From my understanding, ram_console/persistent_ram functionality isn't
really the same as pstore/ramoops.

When asked earlier about this, Colin said:
"pstore is just an interface to write to a persistent storage, with a
tiny driver on it to dump console messages after a panic.  With the
number of crashes we see on ARM cpus that are not panics (mostly
watchdog resets), logging only after a panic is not appropriate, so it
can't replace ramconsole.  persistent_ram could easily be a pstore
driver, but that would not give any new useful functionality, since
ramconsole is better for us than pstore_dump.

Looking at ramoops, it is exactly the same as pstore on ioremapped
ram, but was originally implemented separately.  I see patches now
that convert it to a pstore backend, which leaves us in the same state
as above - only logging panics."

As for the long term merging story, I think Colin's idea of using
persistent_ram a backing for pstore, then adapting pstore as needed to
provide similar continuous console logging like ram_console might be a
way forward, but I don't know of anyone actively working on that atm.

Tony: Do you have any thoughts here?

thanks
-john

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