Re: [RFC] Reliable video POSTing on resume (was: Re: [ACPI] Samsung P35, S3, black screen (radeon))

Nigel Cunningham
2005-02-04T02:51:55Z

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