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Date: Tue, 5 Jun 2018 15:53:25 -0700
From: Brian Norris <briannorris@chromium.org>
To: Ben Greear <greearb@candelatech.com>
Cc: "linux-wireless@vger.kernel.org" <linux-wireless@vger.kernel.org>,
        Arend van Spriel <aspriel@gmail.com>,
        Johannes Berg <johannes.berg@intel.com>
Subject: Re: How to let devcoredump know data has been read?
Message-ID: <20180605225324.GA195207@rodete-desktop-imager.corp.google.com> (sfid-20180606_005334_267586_9E29A02A)
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Hi,

On Tue, Jun 05, 2018 at 03:27:28PM -0700, Ben Greear wrote:
> I have been testing ath10k on 4.16, which uses the devcoredump API
> to notify about dumps.
> 
> I am able to see the binary crash dump at /sys/class/devcoredump/devcd2/data,
> for instance, but if I do another crash quickly, I get no new uevent sent
> and no new crash.
> 
> I see there is a 5 minute timer on the coredump data, but it also seems to indicate
> that if I read the crash, the data should be cleared and ready to be
> recreated again?  How do you notify the system that the crash data has
> been read?
> 
> I tried 'cat' on the data file, but that did not seem to clear anything.

Try *writing* to it?

https://elixir.bootlin.com/linux/v4.16/source/drivers/base/devcoredump.c#L91

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=833c95456a70826d1384883b73fd23aff24d366f

<quote>
The dumped data will be readable in sysfs in the virtual device's
data file under /sys/class/devcoredump/devcd*/. Writing to it will
free the data and remove the device, as does a 5-minute timeout.
</quote>

e.g.:

# ls -l /sys/class/devcoredump/devcd1
lrwxrwxrwx. 1 root root 0 Jun  5 15:49 /sys/class/devcoredump/devcd1 -> ../../devices/virtual/devcoredump/devcd1
# echo 1 > /sys/class/devcoredump/devcd1/data
# ls -l /sys/class/devcoredump/devcd1
ls: cannot access '/sys/class/devcoredump/devcd1': No such file or directory

Unfortunately, devcoredump is a bit lacking in documentation. Arend was
writing a bit for the new trigger mechanism at least.

Brian