arm64: head.S: document the use of callee saved registers

Now that the only remaining occurrences of the use of callee saved
registers are on the primary boot path, add a comment to the code
which register is used for what.

Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index 29a734e..427f6d3 100644 (file)
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -208,6 +208,16 @@ efi_header_end:
 
        __INIT
 
+       /*
+        * The following callee saved general purpose registers are used on the
+        * primary lowlevel boot path:
+        *
+        *  Register   Scope                      Purpose
+        *  x21        stext() .. start_kernel()  FDT pointer passed at boot in x0
+        *  x23        stext() .. start_kernel()  physical misalignment/KASLR offset
+        *  x28        __create_page_tables()     callee preserved temp register
+        *  x19/x20    __primary_switch()         callee preserved temp registers
+        */
 ENTRY(stext)
        bl      preserve_boot_args
        bl      el2_setup                       // Drop to EL1, w0=cpu_boot_mode