perf docs: Correct and clarify jitdump spec

Specification claims latest version of jitdump file format is 2. Current
jit dump reading code treats 1 as the latest version.

Correct spec to match code.

The original language made it unclear the value to be written in the
magic field.

Revise language that the writer always writes the same value. Specify
that the reader uses the value to detect endian mismatches.

Signed-off-by: Steve MacLean <[email protected]>
Acked-by: Stephane Eranian <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Brian Robbins <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Eric Saint-Etienne <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: John Keeping <[email protected]>
Cc: John Salem <[email protected]>
Cc: Leo Yan <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Song Liu <[email protected]>
Cc: Tom McDonald <[email protected]>
Link: http://lore.kernel.org/lkml/BN8PR21MB1362F63CDE7AC69736FC7F9EF7800@BN8PR21MB1362.namprd21.prod.outlook.com
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

tools/perf/Documentation/jitdump-specification.txt

@@ -36,8 +36,8 @@ III/ Jitdump file header format
 Each jitdump file starts with a fixed size header containing the following fields in order:
 
 
-* uint32_t magic     : a magic number tagging the file type. The value is 4-byte long and represents the string "JiTD" in ASCII form. It is 0x4A695444 or 0x4454694a depending on the endianness. The field can be used to detect the endianness of the file
-* uint32_t version   : a 4-byte value representing the format version. It is currently set to 2
+* uint32_t magic     : a magic number tagging the file type. The value is 4-byte long and represents the string "JiTD" in ASCII form. It written is as 0x4A695444. The reader will detect an endian mismatch when it reads 0x4454694a.
+* uint32_t version   : a 4-byte value representing the format version. It is currently set to 1
 * uint32_t total_size: size in bytes of file header
 * uint32_t elf_mach  : ELF architecture encoding (ELF e_machine value as specified in /usr/include/elf.h)
 * uint32_t pad1      : padding. Reserved for future use