ARCv2: don't assume core 0x54 has dual issue

The first release of core4 (0x54) was dual issue only (HS4x).
Newer releases allow hardware to be configured as single issue (HS3x)
or dual issue.

Prevent accessing a HS4x only aux register in HS3x, which otherwise
leads to illegal instruction exceptions
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/include/asm/arcregs.h

@@ -151,6 +151,14 @@ struct bcr_isa_arcv2 {
 #endif
 };
 
+struct bcr_uarch_build_arcv2 {
+#ifdef CONFIG_CPU_BIG_ENDIAN
+	unsigned int pad:8, prod:8, maj:8, min:8;
+#else
+	unsigned int min:8, maj:8, prod:8, pad:8;
+#endif
+};
+
 struct bcr_mpy {
 #ifdef CONFIG_CPU_BIG_ENDIAN
 	unsigned int pad:8, x1616:8, dsp:4, cycles:2, type:2, ver:8;

arch/arc/kernel/setup.c

@@ -199,13 +199,29 @@ static void read_arc_build_cfg_regs(void)
 		cpu->bpu.ret_stk = 4 << bpu.rse;
 
 		if (cpu->core.family >= 0x54) {
+
+			struct bcr_uarch_build_arcv2 uarch;
+
+			/*
+			 * The first 0x54 core (uarch maj:min 0:1 or 0:2) was
+			 * dual issue only (HS4x). But next uarch rev (1:0)
+			 * allows it be configured for single issue (HS3x)
+			 * Ensure we fiddle with dual issue only on HS4x
+			 */
+			READ_BCR(ARC_REG_MICRO_ARCH_BCR, uarch);
+
+			if (uarch.prod == 4) {
 				unsigned int exec_ctrl;
 
+				/* dual issue hardware always present */
+				cpu->extn.dual = 1;
+
 				READ_BCR(AUX_EXEC_CTRL, exec_ctrl);
+
+				/* dual issue hardware enabled ? */
 				cpu->extn.dual_enb = !(exec_ctrl & 1);
 
-			/* dual issue always present for this core */
-			cpu->extn.dual = 1;
+			}
 		}
 	}