Re: [PATCH v3 03/12] bus: mvebu-mbus: Add static window allocation to the DT binding

[Ezequiel Garcia <ezequiel.garcia-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org>]

On Wed, Jun 19, 2013 at 09:08:30PM +0200, Arnd Bergmann wrote:
> On Wednesday 19 June 2013, Ezequiel Garcia wrote:
> > > > What happens is that any decoding window that was setup by the bootloader,
> > > > is wiped and completely new windows are allocated using the translations
> > > > in the DT, as described by this binding.
> > > > 
> > > > This was the case from the start with the old MBus driver. FWIW, I think
> > > > it's actually the best choice that can be made: it makes the kernel
> > > > independent of the previous setting.
> > > > 
> > > > I know you've suggested differently in the past, but I'm not sure I
> > > > understand what's the benefit in keeping the bootloaders configuration.
> > > 
> > > The device tree normally describes things that are either wired up
> > > in hardware or set up by the boot loader. Describing things that the
> > > boot loader may or may not have set up and that the kernel should
> > > set up but may ignore if it wants to is a bit fishy, but it seems
> > > that you have decided to do it that way. You should definitely
> > > document the fact that all ranges except the "internal-regs" are just
> > > suggestions and cannot be relied on to be present at boot time.
> > > 
> > 
> > Hold on! I've just noticed this, and I want to clarify something, just
> > to avoid mis-interpretations. The binding is not saying "the windows
> > described through this ranges are present at boot time".
> > 
> > Rather, it is "this binding will guarantee that the windows described 
> > in it will be present after the mbus allocates them".
> > 
> > Does it sound too fishy?
> 
> I don't think it's a guarantee that the binding can make. The binding
> describes the interface between the hardware/firmware and the kernel,
> not an interface between one kernel driver and another.
> 
> You could instead write:
> 
> "The ranges property defines a set of mbus windows that are expected
> to be set by the operating system and that are guaranteed to be free
> of overlaps with one another or with the system memory ranges.
> Each entry in the property refers to exactly one window. If an
> operating system choses to use a different set of mbus windows,
> it must ensure that any address translations performed from downstream
> devices are adapted accordingly. The operating system may insert
> additional mbus windows that do not conflict with the ones listed
> in the ranges, e.g. for mapping PCIe devices. As a special case,
> the internal register window must be set up by the boot loader
> at the address listed in the ranges property, since the operating
> uses it to set up the other windows."
> 

Nice!

Shamelessly copy-pasted into the binding documentation.

Thanks,
-- 
Ezequiel García, Free Electrons
Embedded Linux, Kernel and Android Engineering
http://free-electrons.com
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