Wednesday, 15 June 2011

Dumping the contents of the Embedded Controller

Dumping the contents of the Embedded Controller (EC) can be useful when debugging some x86 BIOS/kernel related issues.  At a hardware level to get access to the EC memory one goes via the EC command/status and data port.   As a side note, one can determine these ports as follows:

cat /proc/ioports  | grep EC

The preferred way to access these is via the ACPI EC driver in drivers/acpi/ec.c which is used by the ACPI driver to handle read/write operations to the EC memory region.

In addition to this driver, there the ec_sys module that provides a useful debugfs interface to allow one to read + write to the EC memory.  Write support is enabled with the ec_sys module parameter 'write_support' but it is generally discouraged as one may be poking data into memory may break things in an unpredictable manner, hence by default write support is disabled.

So, to dump the contents of the first EC (assuming debugfs is mounted), do:

sudo modprobe ec_sys
sudo od -t x1 /sys/kernel/debug/ec/ec0/io

Simple!

As a bonus, the General Purpose Event bits are also readable from /sys/kernel/debug/ec/ec0/gpe.

Before I stumbled upon ec_sys.c I used a SystemTap script to execute ec_read() in ec.c to do the reading directly.  Yes it's ugly and stupid, but it does prove SystemTap is a very useful tool.

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