Physical memory start address for ARM cores

[Russell King - ARM Linux]

On Mon, May 11, 2015 at 08:47:21PM +0530, Bhaskara rao Budiredla wrote:
> Could you please confirm if ARM architecture mandates the physical
> memory start address to align with a MegaBye boundary (something like
> 0xXXX00000)? I am seeing Linux Kernel crash if I relax that
> restriction.

The ARM architecture doesn't, but we (the kernel) does restrict it for
the sake of cross-machine portability.  See this comment in
arch/arm/boot/compressed.S:

                /*
                 * Find the start of physical memory.  As we are executing
                 * without the MMU on, we are in the physical address space.
                 * We just need to get rid of any offset by aligning the
                 * address.
                 *
                 * This alignment is a balance between the requirements of
                 * different platforms - we have chosen 128MB to allow
                 * platforms which align the start of their physical memory
                 * to 128MB to use this feature, while allowing the zImage
                 * to be placed within the first 128MB of memory on other
                 * platforms.  Increasing the alignment means we place
                 * stricter alignment requirements on the start of physical
                 * memory, but relaxing it means that we break people who
                 * are already placing their zImage in (eg) the top 64MB
                 * of this range.
                 */

We will never change this requirement, as changing it breaks existing
platforms, and that's not allowed.

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