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>

1 files changed, 48 insertions, 10 deletions

diff --git a/arch/parisc/kernel/syscall.S b/arch/parisc/kernel/syscall.S
index 69b6eebc466e..59fc1a43ec3e 100644
--- 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