[PATCH] arm64: expose kernel page size in Image header flags

[Ard Biesheuvel]

In order for a bootloader to be able to decide whether a certain
arm64 kernel image is compatible with the hardware it is running
on, it needs to be able to find out for which page size the kernel
was built. This is necessary, since the architecture does not
mandate support for all page sizes it defines, and the kernel cannot
boot to a state where it can print a diagnostic if it was built for
a page size that is not supported by the hardware.

So assign two bits in the flags field of the Image header, and set
them according to the build time page size. For backward compatibility,
retain the 0b00 value as 'unspecified'.

Acked-by: Catalin Marinas <catalin.marinas at arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
---
 Documentation/arm64/booting.txt | 3 ++-
 arch/arm64/kernel/image.h       | 4 +++-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/Documentation/arm64/booting.txt b/Documentation/arm64/booting.txt
index 1690350f16e7..46056e7d766e 100644
--- a/Documentation/arm64/booting.txt
+++ b/Documentation/arm64/booting.txt
@@ -104,7 +104,8 @@ Header notes:
 - The flags field (introduced in v3.17) is a little-endian 64-bit field
   composed as follows:
   Bit 0: 	Kernel endianness.  1 if BE, 0 if LE.
-  Bits 1-63:	Reserved.
+  Bits 1-2:	Kernel page size.   0=unspecified, 1=4K, 2=16K, 3=64K
+  Bits 3-63:	Reserved.
 
 - When image_size is zero, a bootloader should attempt to keep as much
   memory as possible free for use by the kernel immediately after the
diff --git a/arch/arm64/kernel/image.h b/arch/arm64/kernel/image.h
index 8fae0756e175..5def289bda84 100644
--- a/arch/arm64/kernel/image.h
+++ b/arch/arm64/kernel/image.h
@@ -47,7 +47,9 @@
 #define __HEAD_FLAG_BE	0
 #endif
 
-#define __HEAD_FLAGS	(__HEAD_FLAG_BE << 0)
+#define __HEAD_FLAG_PAGE_SIZE ((PAGE_SHIFT - 10) / 2)
+
+#define __HEAD_FLAGS	(__HEAD_FLAG_BE << 0) | (__HEAD_FLAG_PAGE_SIZE << 1)
 
 /*
  * These will output as part of the Image header, which should be little-endian
-- 
1.9.1