Re: [PATCH] RISC-V-fixes: relocate DTB if it's outside memory region

From: Palmer Dabbelt
Date: Thu Apr 28 2022 - 17:48:18 EST

On Thu, 24 Mar 2022 02:37:29 PDT (-0700), Conor.Dooley@xxxxxxxxxxxxx wrote:

On 22/03/2022 13:28, Nick Kossifidis wrote:
In case the DTB provided by the bootloader/BootROM is before the kernel
image or outside /memory, we won't be able to access it through the
linear mapping, and get a segfault on setup_arch(). Currently OpenSBI
relocates DTB but that's not always the case (e.g. if FW_JUMP_FDT_ADDR
is not specified), and it's also not the most portable approach since
the default FW_JUMP_FDT_ADDR of the generic platform relocates the DTB
at a specific offset that may not be available. To avoid this situation
copy DTB so that it's visible through the linear mapping.

Signed-off-by: Nick Kossifidis <mick@xxxxxxxxxxxx>

Albeit in a backport, I tested this on a PolarFire SoC based board.
So I guess, Tested-by: Conor Dooley <conor.dooley@xxxxxxxxxxxxx>

My scripts don't pick these up unless they're on their own line, not sure if that's too conservative but it's on purpose. I just sort of stumbled into this one so it's in.

And a lot cleaner than using create_pgd_mapping in setup_vm_final to do it :)


arch/riscv/mm/init.c | 21 +++++++++++++++++++--
1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index 0d588032d..697a9aed4 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -206,8 +206,25 @@ static void __init setup_bootmem(void)
* early_init_fdt_reserve_self() since __pa() does
* not work for DTB pointers that are fixmap addresses
- memblock_reserve(dtb_early_pa, fdt_totalsize(dtb_early_va));
+ /*
+ * In case the DTB is not located in a memory region we won't
+ * be able to locate it later on via the linear mapping and
+ * get a segfault when accessing it via __va(dtb_early_pa).
+ * To avoid this situation copy DTB to a memory region.
+ * Note that memblock_phys_alloc will also reserve DTB region.
+ */
+ if (!memblock_is_memory(dtb_early_pa)) {
+ size_t fdt_size = fdt_totalsize(dtb_early_va);
+ phys_addr_t new_dtb_early_pa = memblock_phys_alloc(fdt_size, PAGE_SIZE);
+ void *new_dtb_early_va = early_memremap(new_dtb_early_pa, fdt_size);
+ memcpy(new_dtb_early_va, dtb_early_va, fdt_size);
+ early_memunmap(new_dtb_early_va, fdt_size);
+ _dtb_early_pa = new_dtb_early_pa;
+ } else
+ memblock_reserve(dtb_early_pa, fdt_totalsize(dtb_early_va));
+ }