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..59fc1a43 100644
--- a/arch/parisc/kernel/syscall.S
+++ b/arch/parisc/kernel/syscall.S
@@ -365,17 +365,51 @@
/*********************************************************
- 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 @@
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 @@
/*
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