CODEWITHSUNDEEP
androidlinux-kernelkernellinux-device-driverdevice-driver
TheLoneJoker
TheLoneJoker

Reputation: 1629

Linux apanic mechanism and difference from newer ram_console/persistent_ram mechanism

I am studying the different logging mechanisms on Linux in case of a Kernel panic (OOPS). So far from my search on the web, I have been able to derive the following information:

Could someone enlighten me about the following in this regard ?

I am just looking for pointers and high level answers, in some sense a direction which I can take to research the mechanisms further. I would appreciate any relevant information in this regard.

Thanks

Upvotes: 2

Views: 1867

Answers (1)

yuvaeasy
yuvaeasy

Reputation: 833

Find answers below for your series of questions.

What was the old apanic mechanism, at a high-level how did this work?

It is a mechanism used by Android for message logging. It writes the log message into some specific offset in mtd partition of NAND. The kernel module exports file named called "apanic" to the "debugfs". The debugfs can be mounted on some directory using the below command. " mount -t debugfs none /sys/kernel/debug" which requires debugfs configuration enabled on linux kernel. For more information on this go through the source code in link below.

https://android.googlesource.com/kernel/msm.git/+/android-msm-2.6.35/drivers/misc/apanic.c

Why was the apanic mechanism deprecated, in favor of the ram_console/persistent_ram mechanism ?

This apanic driver is not part of a linux kernel. It is specific to android. It is deprecated in latest version of androids. The reason could be writing into nand partition may not be good idea when kernel opps happens. Since the nand_write is slow as well as has limited write cycles. This is just my opinion. But i am not sure of the actual reason .

What advantages does the new ram_console/persistent_ram mechinsm bring to the table?

It is also a logging mechanism used by android for logging console messages. Here the logging messages goes into RAM area. In case of persistent_ram , the log messages goes to persistent memory area (which will be retained ) of RAM. This driver module creates a file named "last_kmsg" in the proc directory. This message can be read in next boot if the kernel oops and stops booting. For more information go through the source code given in the below link.

https://android.googlesource.com/kernel/common.git/+/android-3.4/drivers/staging/android/ram_console.c

In what way is the ram_console/persistent_ram functionally different from the old apanic approach ?

The former is ram based and the later is mtd based.

In the latest experimental android source i am not finding the source of both. Do not know the reason.

https://android.googlesource.com/kernel/common.git/+/experimental/android-3.10-rc5/drivers/staging/android/

Hope i clarified your queries.

Regards,

Yuvaraj.A

Upvotes: 5

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