[RFC PATCH 09/12] riscv: Allow user to downgrade to sv39 when hw supports sv48

[Alexandre Ghiti <alex@ghiti.fr>]

This is made possible by using the mmu-type property of the cpu node of
the device tree.

By default, the kernel will boot with 4-level page table if the hw supports
it but it can be interesting for the user to select 3-level page table as
it is less memory consuming and faster since it requires less memory
accesses in case of a TLB miss.

Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Reviewed-by: Anup Patel <anup@brainfault.org>
---
 arch/riscv/mm/init.c | 26 ++++++++++++++++++++++++--
 1 file changed, 24 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index cb23a30d9af3..f9a99cb1870b 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -550,10 +550,32 @@ void disable_pgtable_l4(void)
  * then read SATP to see if the configuration was taken into account
  * meaning sv48 is supported.
  */
-asmlinkage __init void set_satp_mode(uintptr_t load_pa)
+asmlinkage __init void set_satp_mode(uintptr_t load_pa, uintptr_t dtb_pa)
 {
 	u64 identity_satp, hw_satp;
+	int cpus_node;
 
+	/* 1/ Check if the user asked for sv39 explicitly in the device tree */
+	cpus_node = fdt_path_offset((void *)dtb_pa, "/cpus");
+	if (cpus_node >= 0) {
+		int node;
+
+		fdt_for_each_subnode(node, (void *)dtb_pa, cpus_node) {
+			const char *mmu_type = fdt_getprop((void *)dtb_pa, node,
+							"mmu-type", NULL);
+			if (!mmu_type)
+				continue;
+
+			if (!strcmp(mmu_type, "riscv,sv39")) {
+				disable_pgtable_l4();
+				return;
+			}
+
+			break;
+		}
+	}
+
+	/* 2/ Determine if the HW supports sv48: if not, fallback to sv39 */
 	create_pgd_mapping(early_pg_dir, load_pa, (uintptr_t)early_pud,
 			   PGDIR_SIZE, PAGE_TABLE);
 	create_pud_mapping(early_pud, load_pa, (uintptr_t)early_pmd,
@@ -611,7 +633,7 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
 #endif
 
 #if defined(CONFIG_64BIT) && !defined(CONFIG_MAXPHYSMEM_2GB)
-	set_satp_mode(load_pa);
+	set_satp_mode(load_pa, dtb_pa);
 #endif
 
 	kernel_virt_addr = KERNEL_VIRT_ADDR;
-- 
2.20.1