[PATCH 4/4] arm64: align PHYS_OFFSET to block size

[Catalin Marinas]

On Mon, Mar 30, 2015 at 04:00:31PM +0200, Ard Biesheuvel wrote:
> On 30 March 2015 at 15:49, Catalin Marinas <catalin.marinas at arm.com> wrote:
> > Can we defer the setting of PHYS_OFFSET until we parse the DT memory
> > nodes?
> 
> I experimented a bit with that, but it is quite hairy. Any
> manipulation of the page tables goes through __va/__pa, so you need a
> valid PHYS_OFFSET there to ensure they point at the right physical
> region.

Yes, so we need set PHYS_OFFSET before we start using __va/__pa.

> But PHYS_OFFSET also needs to be small enough for the DT
> parsing code not to disregard regions that are below it.

With PHYS_OFFSET as 0 initially, no regions would be dropped. But we
could write a simplified early_init_dt_add_memory_arch() which avoids
the PHYS_OFFSET check while setting the actual PHYS_OFFSET to the
minimal address detected. I can see the generic function only uses
__pa() to get the PHYS_OFFSET.

> And then
> there is the memblock limit to ensure that early dynamically allocated
> page tables come from a region that is already mapped.

By the time we start using memblock allocations, we have a PHYS_OFFSET
set. The early DT parsing does not require any memory allocations AFAIK.
We need to make sure that setup_machine_fdt creates a VA mapping of the
DT and does not require the __va() macro (I thought I've seen some
patches to use fixmap for DT).

> I think it may be doable, but it would require some significant
> hacking, e.g., call early_init_scan_dt() at its physical address with
> only the ID map loaded and the MMU and caches on, and only after that
> start populating the virtual address space. Or at least only populate
> the lower half, i.e., mappings below PAGE_OFFSET for the kernel and
> the FDT

With some form of your patches, we already decouple the PAGE_OFFSET from
the kernel text mapping. We map the latter at some very high
KERNEL_PAGE_OFFSET, the DT via fixmap (which is part of the kernel data
section, so mapped at the high KERNEL_PAGE_OFFSET). Once we start
calling early_init_dt_add_memory_arch(), we set the real PAGE_OFFSET and
are free to use __pa/__va after setup_machine_fdt(). The actual linear
mappings will be created later via paging_init().

-- 
Catalin