On Wed, Aug 13, 2014 at 05:31:11PM +0100, Nicolas Pitre wrote:

On Wed, 13 Aug 2014, Lorenzo Pieralisi wrote:

quoted

On most common ARM systems, the low-power states a CPU can be put into are
not discoverable in HW and require device tree bindings to describe
power down suspend operations and idle states parameters.

In order to enable DT based idle states and configure idle drivers, this
patch implements the bulk infrastructure required to parse the device tree
idle states bindings and initialize the corresponding CPUidle driver states
data.

The parsing API accepts a start index that defines the first idle state
that should be initialized by the parsing code in order to give new and
legacy driver flexibility over which states should be parsed using the
new DT mechanism.

The idle states list is obtained from the first cpu in the driver
cpumask, which implicitly means the parsing code expects idle states
(and related list of phandles) to be the same for all CPUs in the
CPUidle driver mask. The kernel does not check this assumption, it must
be enforced by the bootloader to ensure correct system behaviour.

Can we make the kernel a little less reliant on bootloader to ensure 
correct system behaviour please?  If assumptions are assumed by the 
kernel, it should at least print a warning and simply ignore the 
information when such assumption are not respected.

I think the check adds complexity (it means stashing the idle states
phandles for the first cpu, loop through the cpus in the mask and compare
the phandles to the ones stashed for the first cpu for all cpus in the
driver mask) for not much.

I was told that it is not up to the kernel to validate the DT, but if
you want I can implement the check even though I really think it is
overkill.

quoted

+	/*
+	 * We get the idle states for the first logical cpu in the
+	 * driver mask. The kernel does not check idle states on all
+	 * cpus in the driver mask, they are assumed to be the same
+	 * by default.
+	 */

What if they're not?

It boils down to detecting what CPUs share the same idle states and
initialize dynamically a number of CPU idle drivers accordingly. It is
doable on top of this patch which is usable on homogeneous machines as a
starting point.
I do not think it is a big issue to live with this limitation at the moment,
given that it can be augmented when this code gets in the kernel. I did not
want to add more complexity to this already complex patch series, that's
the reason why I have not implemented the multiple driver (whose first
target is obviously bL systems) approach from the beginning, and I think
that's reasonable.

Thanks,
Lorenzo