parisc: document light weight syscall ABI

Document the LWS ABI including implementation notes for
userspace, and comment cleanup.

Remove extraneous .align 16 after lws_lock_start.

Signed-off-by: Carlos O'Donell <carlos@systemhalted.org>
Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Kyle McMartin <kyle@mcmartin.ca>

diff --git a/arch/parisc/kernel/syscall.S b/arch/parisc/kernel/syscall.S
index 69b6eeb..59fc1a4 100644 (file)
--- a/arch/parisc/kernel/syscall.S
+++ b/arch/parisc/kernel/syscall.S
@@ -365,17 +365,51 @@ tracesys_sigexit:
 
 
        /*********************************************************
-               Light-weight-syscall code
+               32/64-bit Light-Weight-Syscall ABI
 
-               r20 - lws number
-               r26,r25,r24,r23,r22 - Input registers
-               r28 - Function return register
-               r21 - Error code.
+               * - Indicates a hint for userspace inline asm
+               implementations.
 
-               Scracth: Any of the above that aren't being
-               currently used, including r1. 
+               Syscall number (caller-saves)
+               - %r20
+               * In asm clobber.
 
-               Return pointer: r31 (Not usable)
+               Argument registers (caller-saves)
+               - %r26, %r25, %r24, %r23, %r22
+               * In asm input.
+
+               Return registers (caller-saves)
+               - %r28 (return), %r21 (errno)
+               * In asm output.
+
+               Caller-saves registers
+               - %r1, %r27, %r29
+               - %r2 (return pointer)
+               - %r31 (ble link register)
+               * In asm clobber.
+
+               Callee-saves registers
+               - %r3-%r18
+               - %r30 (stack pointer)
+               * Not in asm clobber.
+
+               If userspace is 32-bit:
+               Callee-saves registers
+               - %r19 (32-bit PIC register)
+
+               Differences from 32-bit calling convention:
+               - Syscall number in %r20
+               - Additional argument register %r22 (arg4)
+               - Callee-saves %r19.
+
+               If userspace is 64-bit:
+               Callee-saves registers
+               - %r27 (64-bit PIC register)
+
+               Differences from 64-bit calling convention:
+               - Syscall number in %r20
+               - Additional argument register %r22 (arg4)
+               - Callee-saves %r27.
 
                Error codes returned by entry path:
 
@@ -473,7 +507,8 @@ lws_compare_and_swap64:
        b,n     lws_compare_and_swap
 #else
        /* If we are not a 64-bit kernel, then we don't
-        * implement having 64-bit input registers
+        * have 64-bit input registers, and calling
+        * the 64-bit LWS CAS returns ENOSYS.
         */
        b,n     lws_exit_nosys
 #endif
@@ -635,12 +670,15 @@ END(sys_call_table64)
        /*
                All light-weight-syscall atomic operations 
                will use this set of locks 
+
+               NOTE: The lws_lock_start symbol must be
+               at least 16-byte aligned for safe use
+               with ldcw.
        */
        .section .data
        .align  PAGE_SIZE
 ENTRY(lws_lock_start)
        /* lws locks */
-       .align 16
        .rept 16
        /* Keep locks aligned at 16-bytes */
        .word 1