Re: [PATCHv4 4/4] ARM: versatile: support configuring versatile machine for no-MMU

[Russell King - ARM Linux <linux@armlinux.org.uk>]

On Fri, Jun 22, 2018 at 11:57:32AM -0400, Nicolas Pitre wrote:
> That is no longer a kernel config/build issue if you the system 
> integrator put an XIP kernel on a target with the wrong DT.
> 
> The XIP kernel does require that you provide the actual memory base 
> address as a constant to the build system but that could be extracted 
> directly from the DT data if having a kconfig prompt for that is too 
> unwieldy. The new kconfig macro language in mainline could even be 
> leveraged to do that at config time.

That is actually incorrect - you're too used to the dynamic fixup of
virt_to_phys() and friends now!

Given that XIP is typically used in environments where the kernel text
is very much unmodifyable, we disable CONFIG_ARM_PATCH_PHYS_VIRT.  When
disabled, we use a constant translation, based upon the build-time
CONFIG_PHYS_OFFSET and CONFIG_PAGE_OFFSET settings.

The early assembler relies upon these settings - which must be known
before we enable the MMU.  As such, DT parsing is not available to us
at that point (as we can't run any C code, because we won't be calling
it at the address it was linked for.)

I suppose someone who was sufficiently motivated could write assembly-
based parsing of DT to extract the memory regions and work out what the
right settings are.  Is there really the motivation to do that - there
was a burst of XIP interest a few releases ago but it seems to have
tailed off.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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