Re: [PATCH v4 2/8] OF: Introduce DT overlay support.

[Grant Likely <grant.likely@secretlab.ca>]

On Tue, 20 May 2014 09:38:49 +0200, Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> Hi Grant,
> 
> On Tue, May 20, 2014 at 7:50 AM, Grant Likely <grant.likely@secretlab.ca> wrote:
> >> Why has the overlay system been designed for plugging and unpluging whole
> >> overlays?
> >> That means the kernel has to remember the full stack, causing issues with
> >> e.g. kexec.
> >
> > Mostly so that drivers don't see any difference in the livetree data
> > structure. It also means that userspace sees a single representation of
> > the hardware at any given time.
> 
> Sorry, I don't follow the argument about the "single representation of the
> hardware".

Er, s/of the hardware/of the tree/. Right now the overlay design
modifies the live tree which at the same time modifies the tree
representation in /sys/firmware/devicetree. If the design was changed to
keep the overlay logically separate, then I would think we want to
expose that information to usespace also. In fact, I think we would need
to for usecases like kexec.

> 
> >> Why not allowing the addition of removal of subtrees of the full device
> >> tree?
> >
> > Overlays is more than just a subtree. A single overlay can make
> > manipulations of multiple subtrees that should be handled as logically
> > atomic.
> 
> Sure, it's more complicated due to the atomicity of multiple changes.
> 
> >> This is similar to other hotpluggable subsystems (which are not necessarily
> >> DT-based), like PCI Express. That way the kernel can pass a
> >> DT-representation of the actual current device tree to a kexec'ed kernel.
> >
> > I'm not following you argument. Hardware hotplug systems like PCIe don't
> > manipulate the firmware data. The kernel detects the new device and
> > populates the Linux device model directly. Firmware provided data (ACPI
> > or FDT) isn't involved.
> 
> I mean the kernel doesn't remember the exact order in a stack, to reverse
> operations. E.g. I can add hotplug a PCIe bridge with multiple devices
> behind it, and unplug a single device later. It's still one subtree, though.

The problem comes when one overlay adds a node or configuration that is
depended on by the second overlay, but there isn't a direct reference by
the second overlay into the first.

g.