[Buildroot] [PATCH v2] arch/arc: Accommodate 16 KiB MMU pages

[Alexey Brodkin]

ARC processors are known for its configurability and one of those
configurable things is MMU page size which might be set to any
power of two from 4 KiB to 16 MiB, though in the Linux kernel we
only support 4, 8 and 16 KiB due to practical considerations.

And the most used setting is 8 KiB thus GNU LD assumes maximum
page size is 8 KiB by default and while this works for smaller
pages (it's OK to align segments by larger value it will be still
peoperly aligned) this breaks execution of user-space apps on HW
with larger pages because Elf sections might very well span across
allocated pages and thus make executable broken.

Simplest example:
------------------------------------>8-----------------------------------
$ arc-linux-gcc test.c
$ arc-linux-readelf --segments a.out

Program Headers:
  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
...
  LOAD           0x000000 0x00010000 0x00010000 0x003e8 0x003e8 R E 0x2000 <-- See
  LOAD           0x001f24 0x00013f24 0x00013f24 0x000f0 0x0010c RW  0x2000
------------------------------------>8-----------------------------------

Fortunately we may override default page size settings with "max-page-size"
linker option this way:
------------------------------------>8-----------------------------------
$ arc-linux-gcc test.c -Wl,-z,max-page-size=16384
$ arc-linux-readelf --segments a.out
Elf file type is EXEC (Executable file)
Entry point 0x102c4
There are 8 program headers, starting at offset 52

Program Headers:
  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
...
  LOAD           0x000000 0x00010000 0x00010000 0x003e8 0x003e8 R E 0x4000 <-- See
  LOAD           0x001f24 0x00015f24 0x00015f24 0x000f0 0x0010c RW  0x4000
------------------------------------>8-----------------------------------

Which we implement with that change.

Signed-off-by: Alexey Brodkin <abrodkin at synopsys.com>
---

Changes v1 -> v2:

 * Obviously we want to append lenker settings to the wrapper
   but not overwrite atomic option if is set as these are orthogonal
   things.

 arch/arch.mk.arc | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/arch.mk.arc b/arch/arch.mk.arc
index 5fcffb7f4b..d45299144c 100644
--- a/arch/arch.mk.arc
+++ b/arch/arch.mk.arc
@@ -1,4 +1,14 @@
+ifeq ($(BR2_arc),y)
+
 # -matomic is always required when the ARC core has the atomic extensions
-ifeq ($(BR2_arc)$(BR2_ARC_ATOMIC_EXT),yy)
+ifeq ($(BR2_ARC_ATOMIC_EXT),y)
 ARCH_TOOLCHAIN_WRAPPER_OPTS = -matomic
 endif
+
+# By default MAXPAGESIZE for ARC is 8196 so for larger MMU pages
+# it needs to be overridden.
+ifeq ($(BR2_ARC_PAGE_SIZE_16K),y)
+ARCH_TOOLCHAIN_WRAPPER_OPTS += -Wl,-z,max-page-size=16384
+endif
+
+endif
-- 
2.16.2