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Subject[RFC] dev_acpi: device driver for userspace access to ACPI
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This is by no means ready for release, but I wanted to get a sanity
check. I'm still stuck on this idea that userspace needs access to ACPI
namespace. Manageability apps might use this taking inventory of
devices not exposed by other means, things like X can locate chipset
components that don't live in PCI space, there's even the possibility of
making user space drivers.

Populating the sysfs tree didn't seem to generate as much interest as
I'd hoped and I don't think it kept with the spirit of sysfs very well.
So, now I present dev_acpi (name suggestions welcome). The link below
is a tarball with a first stab at the driver as well as a simple proof
of concept application. It should build against any 2.6 kernel as long
as you have the include files available. There are no kernel changes
required, thus it doesn't expose anything not already exposed as a
symbol.

The basic concept of operation is that the ioctl operates on the ACPI
path passed into the ioctl call. The ioctl may return the result of the
operation either in the status field of the argument or use that to
indicate the number of bytes available to read(2) for the result. The
header file included describes the input and output for each operation.
If the status field indicates a byte count to read, the calling
application can easily size buffers, and call read(2) on the device file
to get the results. I've also include support for write(2) that could
allow writing arguments for method calls that take input (completely
untested). I've limited some of the output (for instance in GET_NEXT)
to try to only print out standard ACPI objects, but the filter is pretty
simple (objects beginning w/ '_'). I know the completely open interface
from the sysfs implementation scared some people. Non-standard objects
can still be operated on, but you've got to know what to look for.

Many of the ioctls mimic the behavior of the acpi calls that are
already exported. What I have now is only a start at what could be
provided. The sample, proof-of-concept app, is called acpitree. It's
much like the tree app for listing files and directories. It does
evaluate and print _HIDs represented by integers, but that's about it.
Here's some sample output (sorry anyone not using fixed width fonts):

>From an rx4640 ia64 server:
\\
|-- _GPE
|-- _PR_
|-- _SB_
| |-- SBA0
| | |-- _HID (HWP0001)
| | |-- _CID
| | |-- _CRS
| | |-- _INI
| | |-- _UID
| | |-- MI0_
| | | |-- _HID (IPI0001)
| | | |-- _UID
| | | |-- _STA
| | | `-- _CRS
| | |-- PCI0
| | | |-- _UID
| | | |-- _STA
| | | |-- _BBN
| | | |-- _HID (HWP0002)
| | | |-- _CID
| | | |-- _PRT
...

>From an nc6000 laptop:
\\
|-- _GPE
|-- _PR_
|-- _SB_
| |-- _INI
| |-- C00C
| | |-- _HID (PNP0C01)
| | `-- _CRS
| |-- C046
| | |-- _HID (PNP0A03)
| | |-- _ADR
| | |-- C047
| | | |-- _ADR
| | | |-- C0D1
| | | | |-- _ADR
| | | | |-- _REG
| | | | |-- _S3D
| | | | |-- _S4D
| | | | |-- _DOS
| | | | |-- C0DD
| | | | | |-- _ADR
...

You can find the driver and sample app here:

http://free.linux.hp.com/~awilliam/acpi/dev_acpi/dev_acpi-20040803.tar.bz2

There's a brutally short README there. Caveat: the driver is hardcoded
to use an experimental major number, you'll have to mknod it, see the
README.

Please try it out, let me know if it sucks. I make no guarantees it
won't kill your system, but it shouldn't unless you start evaluating
dangerous objects (ie, if you don't know what it does, don't do it).
And of course, if you have any suggestions, I welcome feedback. Thanks,

Alex

--
Alex Williamson HP Linux & Open Source Lab

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