mips: optimize mips_ejtag_step_disable() code

The code is a bit large compared to mips_ejtag_step_enable().
With the mips32 xori instruction the code can be
reused.
The number of pracc accesses are reduced from 18 to 7.

Change-Id: If3974ebd64da4461c22b089796646990e68e1b72
Signed-off-by: Salvador Arroyo <sarroyofdez@yahoo.es>
Reviewed-on: http://openocd.zylin.com/944
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

src/target/mips32.h

@@ -144,6 +144,7 @@ struct mips32_algorithm {
 #define MIPS32_OP_SH	0x29
 #define MIPS32_OP_SW	0x2B
 #define MIPS32_OP_ORI	0x0D
+#define MIPS32_OP_XORI	0x0E
 #define MIPS32_OP_XOR	0x26
 #define MIPS32_OP_SLTU  0x2B
 #define MIPS32_OP_SRL	0x03
@@ -186,6 +187,7 @@ struct mips32_algorithm {
 #define MIPS32_MTLO(reg)				MIPS32_R_INST(0, reg, 0, 0, 0, MIPS32_OP_MTLO)
 #define MIPS32_MTHI(reg)				MIPS32_R_INST(0, reg, 0, 0, 0, MIPS32_OP_MTHI)
 #define MIPS32_ORI(tar, src, val)		MIPS32_I_INST(MIPS32_OP_ORI, src, tar, val)
+#define MIPS32_XORI(tar, src, val)		MIPS32_I_INST(MIPS32_OP_XORI, src, tar, val)
 #define MIPS32_RDHWR(tar, dst)			MIPS32_R_INST(MIPS32_OP_SPECIAL3, 0, tar, dst, 0, MIPS32_OP_RDHWR)
 #define MIPS32_SB(reg, off, base)		MIPS32_I_INST(MIPS32_OP_SB, base, reg, off)
 #define MIPS32_SH(reg, off, base)		MIPS32_I_INST(MIPS32_OP_SH, base, reg, off)

src/target/mips_ejtag.c

@@ -190,49 +190,32 @@ void mips_ejtag_drscan_8_out(struct mips_ejtag *ejtag_info, uint8_t data)
 	jtag_add_dr_scan(tap, 1, &field, TAP_IDLE);
 }
 
-static int mips_ejtag_step_enable(struct mips_ejtag *ejtag_info)
+/* Set (to enable) or clear (to disable stepping) the SSt bit (bit 8) in Cp0 Debug reg (reg 23, sel 0) */
+int mips_ejtag_config_step(struct mips_ejtag *ejtag_info, int enable_step)
 {
-	static const uint32_t code[] = {
-			MIPS32_MTC0(1, 31, 0),			/* move $1 to COP0 DeSave */
-			MIPS32_MFC0(1, 23, 0),			/* move COP0 Debug to $1 */
-			MIPS32_ORI(1, 1, 0x0100),		/* set SSt bit in debug reg */
-			MIPS32_MTC0(1, 23, 0),			/* move $1 to COP0 Debug */
-			MIPS32_B(NEG16(5)),
-			MIPS32_MFC0(1, 31, 0),			/* move COP0 DeSave to $1 */
-	};
+	int code_len = enable_step ? 6 : 7;
 
-	return mips32_pracc_exec(ejtag_info, ARRAY_SIZE(code), code,
-			0, NULL, 0, NULL, 1);
-}
+	uint32_t *code = malloc(code_len * sizeof(uint32_t));
+	if (code == NULL) {
+		LOG_ERROR("Out of memory");
+		return ERROR_FAIL;
+	}
+	uint32_t *code_p = code;
 
-static int mips_ejtag_step_disable(struct mips_ejtag *ejtag_info)
-{
-	static const uint32_t code[] = {
-			MIPS32_MTC0(15, 31, 0),							/* move $15 to COP0 DeSave */
-			MIPS32_LUI(15, UPPER16(MIPS32_PRACC_STACK)),	/* $15 = MIPS32_PRACC_STACK */
-			MIPS32_ORI(15, 15, LOWER16(MIPS32_PRACC_STACK)),
-			MIPS32_SW(1, 0, 15),							/* sw $1,($15) */
-			MIPS32_SW(2, 0, 15),							/* sw $2,($15) */
-			MIPS32_MFC0(1, 23, 0),							/* move COP0 Debug to $1 */
-			MIPS32_LUI(2, 0xFFFF),							/* $2 = 0xfffffeff */
-			MIPS32_ORI(2, 2, 0xFEFF),
-			MIPS32_AND(1, 1, 2),
-			MIPS32_MTC0(1, 23, 0),							/* move $1 to COP0 Debug */
-			MIPS32_LW(2, 0, 15),
-			MIPS32_LW(1, 0, 15),
-			MIPS32_B(NEG16(13)),
-			MIPS32_MFC0(15, 31, 0),							/* move COP0 DeSave to $15 */
-	};
+	*code_p++ = MIPS32_MTC0(1, 31, 0);			/* move $1 to COP0 DeSave */
+	*code_p++ = MIPS32_MFC0(1, 23, 0),			/* move COP0 Debug to $1 */
+	*code_p++ = MIPS32_ORI(1, 1, 0x0100);			/* set SSt bit in debug reg */
+	if (!enable_step)
+		*code_p++ = MIPS32_XORI(1, 1, 0x0100);		/* clear SSt bit in debug reg */
 
-	return mips32_pracc_exec(ejtag_info, ARRAY_SIZE(code), code,
-		0, NULL, 0, NULL, 1);
-}
+	*code_p++ = MIPS32_MTC0(1, 23, 0);			/* move $1 to COP0 Debug */
+	*code_p++ = MIPS32_B(NEG16((code_len - 1)));		/* jump to start */
+	*code_p = MIPS32_MFC0(1, 31, 0);			/* move COP0 DeSave to $1 */
 
-int mips_ejtag_config_step(struct mips_ejtag *ejtag_info, int enable_step)
-{
-	if (enable_step)
-		return mips_ejtag_step_enable(ejtag_info);
-	return mips_ejtag_step_disable(ejtag_info);
+	int retval = mips32_pracc_exec(ejtag_info, code_len, code, 0, NULL, 0, NULL, 1);
+
+	free(code);
+	return retval;
 }
 
 int mips_ejtag_enter_debug(struct mips_ejtag *ejtag_info)