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-rw-r--r--arch/sh/kernel/entry-common.S21
1 files changed, 15 insertions, 6 deletions
diff --git a/arch/sh/kernel/entry-common.S b/arch/sh/kernel/entry-common.S
index 13047a4facd2..c001f782c5f1 100644
--- a/arch/sh/kernel/entry-common.S
+++ b/arch/sh/kernel/entry-common.S
@@ -268,20 +268,29 @@ debug_trap:
* Syscall #: R3
* Arguments #0 to #3: R4--R7
* Arguments #4 to #6: R0, R1, R2
- * TRA: (number of arguments + ABI revision) x 4
+ * TRA: See following table.
*
- * This code also handles delegating other traps to the BIOS/gdb stub
- * according to:
- *
- * Trap number
* (TRA>>2) Purpose
* -------- -------
* 0x00-0x0f original SH-3/4 syscall ABI (not in general use).
* 0x10-0x1f general SH-3/4 syscall ABI.
- * 0x20-0x2f syscall ABI for SH-2 parts.
+ * 0x1f unified SH-2/3/4 syscall ABI (preferred).
+ * 0x20-0x2f original SH-2 syscall ABI.
* 0x30-0x3f debug traps used by the kernel.
* 0x40-0xff Not supported by all parts, so left unhandled.
*
+ * For making system calls, any trap number in the range for the
+ * given cpu model may be used, but the unified trap number 0x1f is
+ * preferred for compatibility with all models.
+ *
+ * The low bits of the trap number were once documented as matching
+ * the number of arguments, but they were never actually used as such
+ * by the kernel. SH-2 originally used its own separate trap range
+ * because several hardware exceptions fell in the range used for the
+ * SH-3/4 syscall ABI.
+ *
+ * This code also handles delegating other traps to the BIOS/gdb stub.
+ *
* Note: When we're first called, the TRA value must be shifted
* right 2 bits in order to get the value that was used as the "trapa"
* argument.