[U-Boot] FDT: alignment of 64-bits integers

Jean-Jacques Hiblot jjhiblot at ti.com
Fri Sep 20 16:18:39 UTC 2019

Hi Simon,

while working with the FDT in the SPL, I came across an alignment issue. 
I have seen 64-bits integers not aligned on a 64-bit boundary, although 
is doesn't happen very often. According to the FDT spec, the requirement 
is to be aligned on 32-bits, so it is OK.

However on ARM64, until the cache is enabled, it is not possible to 
access 64-bits integers not aligned on a 64-bits boundary. This issue 
has been discussed here : 

I have a patch for this but I am not sure, this is the right approach:

diff --git a/include/linux/libfdt_env.h b/include/linux/libfdt_env.h
index e2bf79c7ee..b6b7176a8a 100644
--- a/include/linux/libfdt_env.h
+++ b/include/linux/libfdt_env.h
@@ -15,7 +15,7 @@

  typedef __be16 fdt16_t;
  typedef __be32 fdt32_t;
-typedef __be64 fdt64_t;
+typedef __be64 fdt64_t __aligned(4);

The trouble with this patch is that for every fdt64_t the access will be 
aligned on 32-bits, inducing an overall performance penalty.

  The other approach will be to fix the problem where it happens. So far 
I had the issue only ofnode_read_u64() because that is what the driver I 
worked with is using.

What do you think?



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