ARCv2: Accomodate HS48 MMUv5 by relaxing MMU ver checking

HS48 cpus will have a new MMUv5, although Linux is currently not
explicitly supporting the newer features (so remains at V4).
The existing software/hardware version check is very tight and causes
boot abort. Given that the MMUv5 hardware is backwards compatible,
relax the boot check to allow current kernel support level to work
with new hardware.

Also while at it, move the ancient MMU related code to under ARCompact
builds as baseline MMU for HS cpus is v4.
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/mm/tlb.c

@@ -762,6 +762,7 @@ void read_decode_mmu_bcr(void)
 	tmp = read_aux_reg(ARC_REG_MMU_BCR);
 	mmu->ver = (tmp >> 24);
 
+	if (is_isa_arcompact()) {
 		if (mmu->ver <= 2) {
 			mmu2 = (struct bcr_mmu_1_2 *)&tmp;
 			mmu->pg_sz_k = TO_KB(0x2000);
@@ -769,7 +770,7 @@ void read_decode_mmu_bcr(void)
 			mmu->ways = 1 << mmu2->ways;
 			mmu->u_dtlb = mmu2->u_dtlb;
 			mmu->u_itlb = mmu2->u_itlb;
-	} else if (mmu->ver == 3) {
+		} else {
 			mmu3 = (struct bcr_mmu_3 *)&tmp;
 			mmu->pg_sz_k = 1 << (mmu3->pg_sz - 1);
 			mmu->sets = 1 << mmu3->sets;
@@ -777,6 +778,7 @@ void read_decode_mmu_bcr(void)
 			mmu->u_dtlb = mmu3->u_dtlb;
 			mmu->u_itlb = mmu3->u_itlb;
 			mmu->sasid = mmu3->sasid;
+		}
 	} else {
 		mmu4 = (struct bcr_mmu_4 *)&tmp;
 		mmu->pg_sz_k = 1 << (mmu4->sz0 - 1);
@@ -818,8 +820,9 @@ int pae40_exist_but_not_enab(void)
 
 void arc_mmu_init(void)
 {
-	char str[256];
 	struct cpuinfo_arc_mmu *mmu = &cpuinfo_arc700[smp_processor_id()].mmu;
+	char str[256];
+	int compat = 0;
 
 	pr_info("%s", arc_mmu_mumbojumbo(0, str, sizeof(str)));
 
@@ -834,15 +837,21 @@ void arc_mmu_init(void)
 	 */
 	BUILD_BUG_ON(!IS_ALIGNED(STACK_TOP, PMD_SIZE));
 
-	/* For efficiency sake, kernel is compile time built for a MMU ver
-	 * This must match the hardware it is running on.
-	 * Linux built for MMU V2, if run on MMU V1 will break down because V1
-	 *  hardware doesn't understand cmds such as WriteNI, or IVUTLB
-	 * On the other hand, Linux built for V1 if run on MMU V2 will do
-	 *   un-needed workarounds to prevent memcpy thrashing.
-	 * Similarly MMU V3 has new features which won't work on older MMU
-	 */
-	if (mmu->ver != CONFIG_ARC_MMU_VER) {
+	/*
+	 * Ensure that MMU features assumed by kernel exist in hardware.
+	 * For older ARC700 cpus, it has to be exact match, since the MMU
+	 * revisions were not backwards compatible (MMUv3 TLB layout changed
+	 * so even if kernel for v2 didn't use any new cmds of v3, it would
+	 * still not work.
+	 * For HS cpus, MMUv4 was baseline and v5 is backwards compatible
+	 * (will run older software).
+	 */
+	if (is_isa_arcompact() && mmu->ver == CONFIG_ARC_MMU_VER)
+		compat = 1;
+	else if (is_isa_arcv2() && mmu->ver >= CONFIG_ARC_MMU_VER)
+		compat = 1;
+
+	if (!compat) {
 		panic("MMU ver %d doesn't match kernel built for %d...\n",
 		      mmu->ver, CONFIG_ARC_MMU_VER);
 	}