linux-kernel - Re: [PATCH 0/5][RFC] Physical PCI slot objects

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Date:	Wed, 14 Nov 2007 10:37:08 -0700
From:	Bjorn Helgaas <bjorn.helgaas@...com>
To:	linux-pci@...ey.karlin.mff.cuni.cz
Cc:	Kristen Carlson Accardi <kristen.c.accardi@...el.com>,
	Greg KH <greg@...ah.com>, Alex Chiang <achiang@...com>,
	gregkh@...e.de, lenb@...nel.org, matthew@....cx,
	rick.jones2@...com, linux-kernel@...r.kernel.org,
	pcihpd-discuss@...ts.sourceforge.net, linux-acpi@...r.kernel.org
Subject: Re: [PATCH 0/5][RFC] Physical PCI slot objects

On Tuesday 13 November 2007 03:59:36 pm Kristen Carlson Accardi wrote:
> On Tue, 13 Nov 2007 12:26:32 -0800 Greg KH <greg@...ah.com> wrote:
> > And isn't there some other tool that dumps the raw ACPI tables?  I
> > thought the acpi developers used it all the time when debugging things
> > with users.
> 
> There are - people should take a look at the Intel Linux Firmware Kit
> for an example of how to parse ACPI tables in userspace - the code
> is also GPL'd, so you are free to use it in another GPL application.
> 
> http://www.linuxfirmwarekit.org/download.php#source
> 
> In many DSDTs I've seen, _SUN is hardcoded anyway and can be found
> by reading the DSDT from userspace.  This is what the firmwarekit
> does to check for duplicate _SUN in one of it's tests.

I see three relevant things in the firmware kit:

  1) ExecuteAmlMethod() in amlpoke/amlpoke.c.  This uses
     /proc/acpi/hotkey/event_config to cause the kernel to
     execute an AML method.  This looks similar to what dev_acpi
     does and is unsafe for the same reasons (the method may have
     side effects that interfere with kernel drivers).  The kernel
     support for this was removed in 2.6.21:

       http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=5ee6edbcde4d3b14e4e03d4b331df1099a34aa8d

  2) execute_aml_method() in acpitable.c.  Similar to above.

  3) parse_SUN_name() in SUN/SUN.c.  This uses acpidump, acpixtract,
     and iasl -d to disassemble the DSDT and SSDTs, then looks for
     things like "Name (_SUN, 0x0000012C)".  That works well if _SUN
     merely returns a constant, and many DSDTs do that.

     But _SUN can be implemented as a control method, and then we have
     a problem because we can't determine the _SUN value by inspecting
     the DSDT.  We have to evaluate the method, and that may require
     operation regions, semaphores, etc., so it can only be done in the
     kernel.

So I agree that the firmware kit has a clever hack that works on much
existing x86 firmware, and it sounds like Tivoli might even rely on
it.  But I don't feel good about it, and it could easily break when
some BIOS writer needs to make _SUN slightly more complicated.

Bjorn
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