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SubjectRe: [RFC] New Driver Model for 2.5
At 10:38 AM -0700 10/18/01, Patrick Mochel wrote:
>On Thu, 18 Oct 2001, Jonathan Lundell wrote:
>> At 11:08 AM -0500 10/18/01, Taral wrote:
>> >On Wed, Oct 17, 2001 at 04:52:29PM -0700, Patrick Mochel wrote:
>> >> When a suspend transition is triggered, the device tree is
>>walked first to
>> >> save the state of all the devices in the system. Once this is
>>complete, the
>> >> saved state, now residing in memory, can be written to some non-volatile
>> >> location, like a disk partition or network location.
> > >>...
> > What happens to state changes between the first and second traversal
>> of the device tree?
>State changes of what?
>After the first walk (save_state), you essentially have a snapshot of the
>system in memory which can be written to disk, memory, etc.
>Once that is done, you disable interrupts and walk the tree again to power
>off devices.

The "state of all the devices in the system". Presumably, while you
walk the tree the first time (to save state) interrupts are enabled,
and devices are active. Operations (including interrupts) on the
device can, presumably, change the state of the device after its
state has been saved.

To take a crude example, suppose you save the state of an Ethernet
NIC, then change its MAC address, and then suspend the device. The
saved state now has the wrong MAC address.

In this particular case, of course, the driver can keep a soft copy
of the current MAC address and and restore from that, but that means
making special cases of special things.

Look at it another way. Why not save the state at the beginning of
time (say when the device is first initialized) instead of walking
the tree at suspend time? Presumably because there's some difference
between the state then, and the state at suspend time. How did that
difference happen, and why couldn't it happen after the save-state
tree-walk but before the actual device suspend?
/Jonathan Lundell.
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