On 05/26/2014 03:36 PM, Sebastian Reichel wrote:
Hi,
On Mon, May 26, 2014 at 10:33:03PM +0100, Grant Likely wrote:
After thinking about it more, I think it is very likely that removing
all the overlays is the correct thing to do in the kexec use-case. When
kexec-ing, it makes sense that we'd want the exact same behaviour from
the kexec'ed kernel. That means we want the device drivers to do the
same thing including loading whatever overlays they depend on.
If the flattened tree was left applied, then the behaviour becomes
different.
I say always remove the overlays unless explicitly told not to, but I'm
struggling to come up with use cases where keeping them applied is
desirable.
I would assume, that I want them applied in most cases. DT describes
the hardware. If I kexec into a new kernel I change software, not
hardware.
Maybe I'm missing the main purpose of the feature. I currently see
two useful usecases for DT overlays:
1. The dtb the kernel is booted with cannot be changed for some
reason, but the board has additional hardware attached (e.g.
the user added a sensor on the i2c bus)
2. The hardware is changed on the fly (e.g. the user flashed the
FPGA part of a zynq processor), sensors on i2c bus, ...
In both cases the kernel should be booted with the additional
overlay information IMHO. Though for the second case it should
be possible to remove the "programmed" hardware information
somehow.
3. Some hot-plug device or card is inserted or removed.
I would argue that the kernel should _not_ be booted with the overlay in place.
Otherwise the code handling overlays would have to have special handling
for the restart case, which is much more complex than just to re-insert
the overlay when it is determined that the device or card is still there.
Guenter
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