[PATCH v2] common: fdt: Remove additional 4k space for fdt allocation

[Stephen Warren]

On 6/11/20 5:10 AM, Michal Simek wrote:
> From: Ashok Reddy Soma <ashok.reddy.soma at xilinx.com>
> 
> There is no technical reason to add additional 4k space for FDT. This space
> is completely unused and just increase memory requirements. This is
> problematic on systems with limited memory resources as Xilinx Zynq
> CSE/ZynqMP mini and Versal mini configurations.

I tried to work out where this additional space came from, but can't
find any obvious clues why it's there.

The extra 4k was originally introduced in x86-specific code by:

commit f697d528caba0c30382b7269fd36f1111e51810d
    x86: Support relocation of FDT on start-up
+int copy_fdt_to_ram(void)
+               fdt_size = ALIGN(fdt_totalsize(gd->fdt_blob) + 0x1000, 32);

... and later copied to generic code by:

commit 1938f4a5b62fc03c52b47697a89b2bb47b77c90c (HEAD)
    Introduce generic pre-relocation board_f.c
+static int reserve_fdt(void)
+               gd->fdt_size = ALIGN(fdt_totalsize(gd->fdt_blob) +
0x1000, 32);

However, there's no obvious comment re: why 4k was added. I wonder if
it's anything to do with 4k==page size and hence different MMU
permissions for the DTB and any data following it? That's purely a guess
though, since I don't see anything in those patches messing with the MMU
any differently for the DTB.

So I guess if this patch passes testing, it's good:-)