Am Donnerstag, 23. September 2004 21:56 schrieb Alan Stern:
> On Thu, 23 Sep 2004, Oliver Neukum wrote:
>
> > Swsusp goes through a sync - suspend - resume - write image - suspend
> > cycle to bring down the machine. There's a certain logic to that because
> > after a reboot all devices will be running.
>
> Can you give me a pointer to the swsusp code?
>
> Also, why the first suspend? Why not sync - resume - write image -
> suspend? After all, any devices that get suspended there will wake up
> immediately during the resume step.
Ok. The meat of swsusp is in kernel/power::swsusp.c.
The call trace of the core is: (comments are mine)
free_some_memory(); // this would require anything that is in the page out path to be
active
device_suspend(3) // this causes device state to be saved in ram
device_power_down(3); // as above, for those that need interrupts off
is_problem = suspend_prepare_image(); //the image is generated
device_power_up(); // wake everything up
suspend_save_image(); // write out the image at least swap device
needs to be active
suspend_power_down(); // kill all power
IMO the crucial part here is the placement of suspend_prepare_image().
It ensures that the system image that will be written is an image of all
devices powered down and all state (like IP assigned to a network interface)
is saved.
The resume will power down the devices again, unpack the image, and
restore all state. I see no reason the first call to device_suspend()
actually needs to do actually switch power levels. As far as I can tell,
the requirements are:
- no DMA
- no interrupts
- saved state
The trick is decoupling taking the image from writing it out.
> > This cannot make a difference. Consider that one of the lists may be
> > empty. I fail to see the logic. Please elaborate. You can have systems
> > where you can shut down nothing without shutting down the swap
> > device.
>
> I _said_ that I don't fully grasp all the implications and requirements
> here.
Neither do I >:->
[..]
> Now let's suppose we're doing the second suspend on your list, the actual
> power-off suspend. Since the memory image has already been written,
> there's nothing wrong with shutting down the swap device.
Actually, it is a requirement.
Regards
Oliver
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