metag: Basic documentation

Add basic metag documentation. This includes an outline description of
the ABIs (including syscall ABI) and calling conventions, similar to the
one in Documentation/frv/.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Rob Landley <rob@landley.net>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: linux-doc@vger.kernel.org

2 files changed, 260 insertions, 0 deletions

diff --git a/Documentation/metag/00-INDEX b/Documentation/metag/00-INDEX
new file mode 100644
index 000000000000..db11c513bd5c
--- /dev/null
+++ b/Documentation/metag/00-INDEX
@@ -0,0 +1,4 @@
+00-INDEX
+	- this file
+kernel-ABI.txt
+	- Documents metag ABI details
diff --git a/Documentation/metag/kernel-ABI.txt b/Documentation/metag/kernel-ABI.txt
new file mode 100644
index 000000000000..7b8dee83b9c1
--- /dev/null
+++ b/Documentation/metag/kernel-ABI.txt
@@ -0,0 +1,256 @@
+			==========================
+			KERNEL ABIS FOR METAG ARCH
+			==========================
+
+This document describes the Linux ABIs for the metag architecture, and has the
+following sections:
+
+ (*) Outline of registers
+ (*) Userland registers
+ (*) Kernel registers
+ (*) System call ABI
+ (*) Calling conventions
+
+
+====================
+OUTLINE OF REGISTERS
+====================
+
+The main Meta core registers are arranged in units:
+
+	UNIT	Type	DESCRIPTION	GP	EXT	PRIV	GLOBAL
+	=======	=======	===============	=======	=======	=======	=======
+	CT	Special	Control unit
+	D0	General	Data unit 0	0-7	8-15	16-31	16-31
+	D1	General	Data unit 1	0-7	8-15	16-31	16-31
+	A0	General	Address unit 0	0-3	4-7	 8-15	 8-15
+	A1	General	Address unit 1	0-3	4-7	 8-15	 8-15
+	PC	Special	PC unit		0		 1
+	PORT	Special	Ports
+	TR	Special	Trigger unit			 0-7
+	TT	Special	Trace unit			 0-5
+	FX	General	FP unit			0-15
+
+GP registers form part of the main context.
+
+Extended context registers (EXT) may not be present on all hardware threads and
+can be context switched if support is enabled and the appropriate bits are set
+in e.g. the D0.8 register to indicate what extended state to preserve.
+
+Global registers are shared between threads and are privilege protected.
+
+See arch/metag/include/asm/metag_regs.h for definitions relating to core
+registers and the fields and bits they contain. See the TRMs for further details
+about special registers.
+
+Several special registers are preserved in the main context, these are the
+interesting ones:
+
+	REG	(ALIAS)		PURPOSE
+	=======================	===============================================
+	CT.1	(TXMODE)	Processor mode bits (particularly for DSP)
+	CT.2	(TXSTATUS)	Condition flags and LSM_STEP (MGET/MSET step)
+	CT.3	(TXRPT)		Branch repeat counter
+	PC.0	(PC)		Program counter
+
+Some of the general registers have special purposes in the ABI and therefore
+have aliases:
+
+	D0 REG	(ALIAS)	PURPOSE		D1 REG	(ALIAS)	PURPOSE
+	===============	===============	===============	=======================
+	D0.0	(D0Re0)	32bit result	D1.0	(D1Re0)	Top half of 64bit result
+	D0.1	(D0Ar6)	Argument 6	D1.1	(D1Ar5)	Argument 5
+	D0.2	(D0Ar4)	Argument 4	D1.2	(D1Ar3)	Argument 3
+	D0.3	(D0Ar2)	Argument 2	D1.3	(D1Ar1)	Argument 1
+	D0.4	(D0FrT)	Frame temp	D1.4	(D1RtP)	Return pointer
+	D0.5		Call preserved	D1.5		Call preserved
+	D0.6		Call preserved	D1.6		Call preserved
+	D0.7		Call preserved	D1.7		Call preserved
+
+	A0 REG	(ALIAS)	PURPOSE		A1 REG	(ALIAS)	PURPOSE
+	===============	===============	===============	=======================
+	A0.0	(A0StP)	Stack pointer	A1.0	(A1GbP)	Global base pointer
+	A0.1	(A0FrP)	Frame pointer	A1.1	(A1LbP)	Local base pointer
+	A0.2				A1.2
+	A0.3				A1.3
+
+
+==================
+USERLAND REGISTERS
+==================
+
+All the general purpose D0, D1, A0, A1 registers are preserved when entering the
+kernel (including asynchronous events such as interrupts and timer ticks) except
+the following which have special purposes in the ABI:
+
+	REGISTERS	WHEN	STATUS		PURPOSE
+	===============	=======	===============	===============================
+	D0.8		DSP	Preserved	ECH, determines what extended
+						DSP state to preserve.
+	A0.0	(A0StP)	ALWAYS	Preserved	Stack >= A0StP may be clobbered
+						at any time by the creation of a
+						signal frame.
+	A1.0	(A1GbP)	SMP	Clobbered	Used as temporary for loading
+						kernel stack pointer and saving
+						core context.
+	A0.15		!SMP	Protected	Stores kernel stack pointer.
+	A1.15		ALWAYS	Protected	Stores kernel base pointer.
+
+On UP A0.15 is used to store the kernel stack pointer for storing the userland
+context. A0.15 is global between hardware threads though which means it cannot
+be used on SMP for this purpose. Since no protected local registers are
+available A1GbP is reserved for use as a temporary to allow a percpu stack
+pointer to be loaded for storing the rest of the context.
+
+
+================
+KERNEL REGISTERS
+================
+
+When in the kernel the following registers have special purposes in the ABI:
+
+	REGISTERS	WHEN	STATUS		PURPOSE
+	===============	=======	===============	===============================
+	A0.0	(A0StP)	ALWAYS	Preserved	Stack >= A0StP may be clobbered
+						at any time by the creation of
+						an irq signal frame.
+	A1.0	(A1GbP)	ALWAYS	Preserved	Reserved (kernel base pointer).
+
+
+===============
+SYSTEM CALL ABI
+===============
+
+When a system call is made, the following registers are effective:
+
+	REGISTERS	CALL			RETURN
+	===============	=======================	===============================
+	D0.0	(D0Re0)				Return value (or -errno)
+	D1.0	(D1Re0)	System call number	Clobbered
+	D0.1	(D0Ar6)	Syscall arg #6		Preserved
+	D1.1	(D1Ar5)	Syscall arg #5		Preserved
+	D0.2	(D0Ar4)	Syscall arg #4		Preserved
+	D1.2	(D1Ar3)	Syscall arg #3		Preserved
+	D0.3	(D0Ar2)	Syscall arg #2		Preserved
+	D1.3	(D1Ar1)	Syscall arg #1		Preserved
+
+Due to the limited number of argument registers and some system calls with badly
+aligned 64-bit arguments, 64-bit values are always packed in consecutive
+arguments, even if this is contrary to the normal calling conventions (where the
+two halves would go in a matching pair of data registers).
+
+For example fadvise64_64 usually has the signature:
+
+	long sys_fadvise64_64(i32 fd, i64 offs, i64 len, i32 advice);
+
+But for metag fadvise64_64 is wrapped so that the 64-bit arguments are packed:
+
+	long sys_fadvise64_64_metag(i32 fd,      i32 offs_lo,
+				    i32 offs_hi, i32 len_lo,
+				    i32 len_hi,  i32 advice)
+
+So the arguments are packed in the registers like this:
+
+	D0 REG	(ALIAS)	VALUE		D1 REG	(ALIAS)	VALUE
+	===============	===============	===============	=======================
+	D0.1	(D0Ar6)	advice		D1.1	(D1Ar5)	hi(len)
+	D0.2	(D0Ar4)	lo(len)		D1.2	(D1Ar3)	hi(offs)
+	D0.3	(D0Ar2)	lo(offs)	D1.3	(D1Ar1)	fd
+
+
+===================
+CALLING CONVENTIONS
+===================
+
+These calling conventions apply to both user and kernel code. The stack grows
+from low addresses to high addresses in the metag ABI. The stack pointer (A0StP)
+should always point to the next free address on the stack and should at all
+times be 64-bit aligned. The following registers are effective at the point of a
+call:
+
+	REGISTERS	CALL			RETURN
+	===============	=======================	===============================
+	D0.0	(D0Re0)				32bit return value
+	D1.0	(D1Re0)				Upper half of 64bit return value
+	D0.1	(D0Ar6)	32bit argument #6	Clobbered
+	D1.1	(D1Ar5)	32bit argument #5	Clobbered
+	D0.2	(D0Ar4)	32bit argument #4	Clobbered
+	D1.2	(D1Ar3)	32bit argument #3	Clobbered
+	D0.3	(D0Ar2)	32bit argument #2	Clobbered
+	D1.3	(D1Ar1)	32bit argument #1	Clobbered
+	D0.4	(D0FrT)				Clobbered
+	D1.4	(D1RtP)	Return pointer		Clobbered
+	D{0-1}.{5-7}				Preserved
+	A0.0	(A0StP)	Stack pointer		Preserved
+	A1.0	(A0GbP)				Preserved
+	A0.1	(A0FrP)	Frame pointer		Preserved
+	A1.1	(A0LbP)				Preserved
+	A{0-1},{2-3}				Clobbered
+
+64-bit arguments are placed in matching pairs of registers (i.e. the same
+register number in both D0 and D1 units), with the least significant half in D0
+and the most significant half in D1, leaving a gap where necessary. Futher
+arguments are stored on the stack in reverse order (earlier arguments at higher
+addresses):
+
+	ADDRESS		0     1     2     3	4     5     6     7
+	===============	===== ===== ===== =====	===== ===== ===== =====
+	A0StP       -->
+	A0StP-0x08	32bit argument #8	32bit argument #7
+	A0StP-0x10	32bit argument #10	32bit argument #9
+
+Function prologues tend to look a bit like this:
+
+	/* If frame pointer in use, move it to frame temp register so it can be
+	   easily pushed onto stack */
+	MOV	D0FrT,A0FrP
+
+	/* If frame pointer in use, set it to stack pointer */
+	ADD	A0FrP,A0StP,#0
+
+	/* Preserve D0FrT, D1RtP, D{0-1}.{5-7} on stack, incrementing A0StP */
+	MSETL	[A0StP++],D0FrT,D0.5,D0.6,D0.7
+
+	/* Allocate some stack space for local variables */
+	ADD	A0StP,A0StP,#0x10
+
+At this point the stack would look like this:
+
+	ADDRESS		0     1     2     3	4     5     6     7
+	===============	===== ===== ===== =====	===== ===== ===== =====
+	A0StP       -->
+	A0StP-0x08
+	A0StP-0x10
+	A0StP-0x18	Old D0.7		Old D1.7
+	A0StP-0x20	Old D0.6		Old D1.6
+	A0StP-0x28	Old D0.5		Old D1.5
+	A0FrP       -->	Old A0FrP (frame ptr)	Old D1RtP (return ptr)
+	A0FrP-0x08	32bit argument #8	32bit argument #7
+	A0FrP-0x10	32bit argument #10	32bit argument #9
+
+Function epilogues tend to differ depending on the use of a frame pointer. An
+example of a frame pointer epilogue:
+
+	/* Restore D0FrT, D1RtP, D{0-1}.{5-7} from stack, incrementing A0FrP */
+	MGETL	D0FrT,D0.5,D0.6,D0.7,[A0FrP++]
+	/* Restore stack pointer to where frame pointer was before increment */
+	SUB	A0StP,A0FrP,#0x20
+	/* Restore frame pointer from frame temp */
+	MOV	A0FrP,D0FrT
+	/* Return to caller via restored return pointer */
+	MOV	PC,D1RtP
+
+If the function hasn't touched the frame pointer, MGETL cannot be safely used
+with A0StP as it always increments and that would expose the stack to clobbering
+by interrupts (kernel) or signals (user). Therefore it's common to see the MGETL
+split into separate GETL instructions:
+
+	/* Restore D0FrT, D1RtP, D{0-1}.{5-7} from stack */
+	GETL	D0FrT,D1RtP,[A0StP+#-0x30]
+	GETL	D0.5,D1.5,[A0StP+#-0x28]
+	GETL	D0.6,D1.6,[A0StP+#-0x20]
+	GETL	D0.7,D1.7,[A0StP+#-0x18]
+	/* Restore stack pointer */
+	SUB	A0StP,A0StP,#0x30
+	/* Return to caller via restored return pointer */
+	MOV	PC,D1RtP