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    SubjectRe: [PATCH 11/13] android: persistent_trace: ftrace into persistent_ram
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    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@android.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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