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SubjectRe: Resume/Suspend (was Re: LVM / Filesystems / High availability)
On Wed, 24 Jun 1998, Magnus Redin wrote:

>
> > Hey, there's a thought! That is a good idea.Almost mainframe like in
> > recovery... X is a no-go with this, though. You need to save the
> > complete state of a SVGA card. Thoughts?
> > -Shawn
>
> It should be enough to ask the X-server to redraw the screen.

Nope, that is not enough with most SVGA cards. Since many VGA
registers are write-only, X has to keep softcopies of those regs in order
to be kept aware of exactly what state the video hardware is in. If you
checkpoint the X server, run some other video card banging program like
SVGALib or Suid-KGI or even another X server, and then restart the old X
server, its register softcopies may not any longer correspond to the
actual state of the hardware = problems.

There is also the issue of atomicity of accelerator commands - a
lot of cards don't take well to being interrupted whilst their accel
command FIFOs are half full of command words. Even if the next bit of
code that hits the video card after X is checkpointed in this case doesn't
cause the card to barf and lock the bus, what happens when the X server
is restarted and finishes filling the FIFOs with the rest of the accel
command words, producing at best a garbled display and at worst DMAing
garbage all over the kernel's address space? Fun times.

If the video drivers were in the kernel, OTOH, you could indeed
just ask the X server to redraw the screen and everything would work OK
since both softcopying and atomicity would be handled by the video
drivers. With LibGGI, you'd just have the restarter send SIGWINCH to the
restarted X server (or any other app) and it would be treated the same way
as if a VT switch had occured and redraw its display.

Jon


---
'Cloning and the reprogramming of DNA is the first serious step in
becoming one with God.'
- Scientist G. Richard Seed


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