[PATCH v6 9/9] ARM: vexpress: Add Device Tree for V2P-CA15 core tile (TC1 variant)

[Grant Likely]

On Thu, Jan 19, 2012 at 11:09 AM, Nicolas Pitre <nico at fluxnic.net> wrote:
> On Thu, 19 Jan 2012, Grant Likely wrote:
>
>> On Thu, Jan 19, 2012 at 10:50 AM, Russell King - ARM Linux
>> <linux at arm.linux.org.uk> wrote:
>> > On Thu, Jan 19, 2012 at 05:27:15PM +0000, Pawel Moll wrote:
>> >> On Thu, 2012-01-19 at 17:00 +0000, David Vrabel wrote:
>> >> > The problem wasn't with including skeleton.dtsi.
>> >>
>> >> Including as it is creates two device_type="memory" nodes, one with
>> >> regs=<0 0>, which is definitely wrong.
>> >>
>> >> > With
>> >> > CONFIG_ARM_ATAG_DTB_COMPAT the zImage decompressor modifies the appended
>> >> > DTB using information from the ATAGs (see atags_to_fdt()).
>> >> >
>> >> > If there's an ATAG giving the amount of RAM the DTB's "memory" node is
>> >> > replaced with a new one.  Since the vexpress DTBs don't have a "memory"
>> >> > node it's added and the DTB ends up with two nodes describing memory.
>> >>
>> >> The "memory at address" node name is in my opinion perfectly legal - p. 3.4
>> >> of the DT spec says "The name component of the node name (see 2.2.1)
>> >> shall be memory.". So the decompressor code may be wrong in looking for
>> >> adress-less "memory" node...
>> >
>> > I don't think you can expect such early code to properly parse a DT tree
>> > with a variability in how memory stuff is declared into that DT tree.
>> >
>> > What if you have two memory nodes specified in the DT file, and the
>> > ATAG data contains one?
>>
>> Yes, just because it is technically legal doesn't make it okay.  The
>> pragmatic approach here is that the skeleton.dtsi file calls the node
>> "memory", so this .dts file must do the same.
>>
>> > The more I look at this, the more I'm convinced that Grant's idea that
>> > DT should entirely override ATAGs all the way to the kernel proper was
>> > the wrong solution - at least in the kernel, if we had both available,
>> > we could make a choice there, and have the full DT library to be able
>> > to manipulate the DT blob.
>>
>> Hey!  I was originally lobbying for the dt pointer carried by an ATAG.
>>  Nico conviced me otherwise.  :-)
>
> Hey!  I was originally lobbying for people to have a fully DT aware
> bootloader if they wanted to play with DT, otherwise there is no
> incentive for updated bootloaders.  But someone else convinced me
> otherwise.  :-)
>
> Mixed bags always have loose ends.

Hahaha.  Having said that though, I still strongly agree with what we
have.  Giving the kernel only one or the other avoids any weirdness
about which the kernel should choose when both are present.  The
bootwrapper atag-to-dt conversion is a migration and development tool,
and it should not do any DT manipulation must be kept to a bare
minimum.

g.