linux-kernel - Re: [PATCH] mtd: mtdoops: optionally dump boottime

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Date:   Mon, 21 May 2018 09:13:09 +0200
From:   Richard Weinberger <richard@....at>
To:     Stefan Schaeckeler <sschaeck@...co.com>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        Brian Norris <computersforpeace@...il.com>,
        Boris Brezillon <boris.brezillon@...tlin.com>,
        Marek Vasut <marek.vasut@...il.com>,
        "linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mtd: mtdoops: optionally dump boottime

Stefan,

Am Montag, 21. Mai 2018, 04:29:24 CEST schrieb Stefan Schaeckeler:
> Hello Richard and others,
> 
> > I get the use-case, but why is this only for mtdoops?
> 
> Powerpc's nvram module also stores oops messages and does so by adding an
> additional timestamp, as well (search for kmsg_dump_get_buffer() in
> arch/powerpc/kernel/nvram_64.c).

Just one more reason to make it generic.
 
> This timestamp is the number of seconds since 1970 and stored as a 64 bit
> integer in the nvram header. Basically, the last kmesg timestamp is a few ms
> less than this additionally stored timestamp. Recording boottime would be more
> elegant, I guess.
> 
> 
> > IMHO this needs to go into generic code such that all kmsg dumpers can
> > benefit from it.
> 
> This would be not that easy:
> 
> #1 kmsg_dump_get_buffer(...size...) returns the most recent <size> bytes.
> Consecutive calls return older chunks. It would be natural to return the
> boottime as the first line, e.g. in the last call, but some clients such as
> mtdoops call kmsg_dump_get_buffer() only once. The returned buffer may be
> complete including boottime, or not.
> 
> #2 consistency with other clients: nvram_64.c has the same requirement of
> storing a kind of wall-time but does it in a completely different way: no
> readable ascii text timestamp preprended to the kmsg buffer but a 64 bit
> timestamp in its header. Note, I don't think we should make mtdoops behave like
> nvram_64.c by storing the timestamp as a 64 bit integer (in its header) b/c
> most people do a cat or string of the mtd device /dev/mtdX and a 64 bit integer
> would just read as garbage.

I think this is the perfect opportunity to talk with other kmesg dump users on
how to add a timestamp.
As last resort we can always do it in our own way but I think making it generic
is at lest worth a try...

> I hope we can have separate implementations for recording additional
> timestamps. Later, I'll send a patch with stylistic changes unless we
> completely disagree on how to move forward.

Both #1 and #2 aren't show-stoppers.

Thanks,
//richard