Subject: Re: [RFC] Reliable video POSTing on resume (was: Re: [ACPI]
	Samsung P35, S3, black screen (radeon))
From: Nigel Cunningham <ncunningham@linuxmail.org>
Reply-To: ncunningham@linuxmail.org
To: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2005@gmx.net>
Cc: Pavel Machek <pavel@ucw.cz>, ACPI List <acpi-devel@lists.sourceforge.net>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Date: Fri, 04 Feb 2005 13:51:55 +1100
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Hi.

On Fri, 2005-02-04 at 13:35, Carl-Daniel Hailfinger wrote:
> Can we use call_usermodehelper at this early resume stage (before any
> video access)? Calling vm86 directly is probably not going to fly
> because we want to be shielded from any misbehaviour in the bios code
> and it may be necessary to kill the process running the bios code.
> 
> Cases in point: My bios will cause the POSTing application to segfault.
> Others have reported the POSTing application just hangs, but the
> important things are done before the hang, so killing it after maybe
> 5 seconds would be ok.

Yuckkkkk!

> Rough outline how to do that without adding tons of code:
> We need a call_usermodehelper_from_ram_with_timeout for that.
> 
> Kernel:                          Userspace:
> User wants to enter S3
> init_mutex_locked(s3_sem)
> call_usermodehelper("vesaposter")
>                                  vesaposter calls LRMI_init
>                                  vesaposter mlocks all its memory
>                                  vesaposter calls into kernel
>                                           and down(s3_sem)
> suspend vesafb
> 
> User has triggered resume
> run wakeup.S
> thaw essential threads
> set a timer of 5 seconds
> up(s3_sem)
> thaw and schedule vesaposter
> wait for timer or vesaposter termination
>                                  vesaposter returns from kernel
>                                  vesaposter posts video card
>                                  vesaposter terminates
> resume vesafb
> continue resume
> 
> Problems with that approach:
> - vesaposter has to be locked in memory to avoid disk accesses

That's no biggie.

> - vesafb has to refrain from touching video until after POST

Which is why I think we should do the post asap (as you have). What is
counted as an essential thread?

> - the vesaposter thread has to be the first unfrozen and
>   scheduled and completed thread. Only after that we can resume
>   vesafb and other threads

All other threads will be frozen, and we don't have scheduler hooks that
will hang up our new kiddie, so we should be right there.

> - the locking will be tricky

But also simplified by the fact that everything else is frozen.

> Advantages:
> - the problems all seem to be solvable easily
> - security and stability are similar to the current userspace code
> - we can use vesafb on such machines
> - the kernel code won't be much more than two dozen lines
> - the userspace POSTing code can be upgraded without the need
>   for kernel updates (no policy in the kernel)
> 
> What do you think?

Show us some code :>

Nigel
-- 
Nigel Cunningham
Software Engineer, Canberra, Australia
http://www.cyclades.com

Ph: +61 (2) 6292 8028      Mob: +61 (417) 100 574

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