csky: Use generic asid algorithm to implement switch_mm

Use linux generic asid/vmid algorithm to implement csky
switch_mm function. The algorithm is from arm and it could
work with SMP system. It'll help reduce tlb flush for
switch_mm in task/vm switch.

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Arnd Bergmann <arnd@arndb.de>

arch/csky/mm/Makefile

@@ -13,3 +13,4 @@ obj-y +=			ioremap.o
 obj-y +=			syscache.o
 obj-y +=			tlb.o
 obj-y +=			asid.o
+obj-y +=			context.o

arch/csky/mm/context.c

@@ -0,0 +1,46 @@
+// SPDX-License-Identifier: GPL-2.0
+// Copyright (C) 2018 Hangzhou C-SKY Microsystems co.,ltd.
+
+#include <linux/bitops.h>
+#include <linux/sched.h>
+#include <linux/slab.h>
+#include <linux/mm.h>
+
+#include <asm/asid.h>
+#include <asm/mmu_context.h>
+#include <asm/smp.h>
+#include <asm/tlbflush.h>
+
+static DEFINE_PER_CPU(atomic64_t, active_asids);
+static DEFINE_PER_CPU(u64, reserved_asids);
+
+struct asid_info asid_info;
+
+void check_and_switch_context(struct mm_struct *mm, unsigned int cpu)
+{
+	asid_check_context(&asid_info, &mm->context.asid, cpu, mm);
+}
+
+static void asid_flush_cpu_ctxt(void)
+{
+	local_tlb_invalid_all();
+}
+
+static int asids_init(void)
+{
+	BUG_ON(((1 << CONFIG_CPU_ASID_BITS) - 1) <= num_possible_cpus());
+
+	if (asid_allocator_init(&asid_info, CONFIG_CPU_ASID_BITS, 1,
+				asid_flush_cpu_ctxt))
+		panic("Unable to initialize ASID allocator for %lu ASIDs\n",
+		      NUM_ASIDS(&asid_info));
+
+	asid_info.active = &active_asids;
+	asid_info.reserved = &reserved_asids;
+
+	pr_info("ASID allocator initialised with %lu entries\n",
+		NUM_CTXT_ASIDS(&asid_info));
+
+	return 0;
+}
+early_initcall(asids_init);