Re: [PATCH v4] ARM: boot: Obtain start of physical memory from DTB

[Ard Biesheuvel <ardb@kernel.org>]

On Fri, 20 Mar 2020 at 15:43, Geert Uytterhoeven
<geert+renesas@glider.be> wrote:
>
> Currently, the start address of physical memory is obtained by masking
> the program counter with a fixed mask of 0xf8000000.  This mask value
> was chosen as a balance between the requirements of different platforms.
> However, this does require that the start address of physical memory is
> a multiple of 128 MiB, precluding booting Linux on platforms where this
> requirement is not fulfilled.
>
> Fix this limitation by obtaining the start address from the DTB instead,
> if available (either explicitly passed, or appended to the kernel).
> Fall back to the traditional method when needed.
>
> This allows to boot Linux on r7s9210/rza2mevb using the 64 MiB of SDRAM
> on the RZA2MEVB sub board, which is located at 0x0C000000 (CS3 space),
> i.e. not at a multiple of 128 MiB.
>
> Suggested-by: Nicolas Pitre <nico@fluxnic.net>
> Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
> Reviewed-by: Nicolas Pitre <nico@fluxnic.net>
> Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
> ---
> v4:
>   - Fix stack location after commit 184bf653a7a452c1 ("ARM:
>     decompressor: factor out routine to obtain the inflated image
>     size"),
>

Apologies for the breakage. I was aware of the existence of this
patch, but I didn't realize it was accessing LC0 early on to find the
stack pointer value.

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>

> v3:
>   - Add Reviewed-by,
>   - Fix ATAGs with appended DTB,
>   - Add Tested-by,
>
> v2:
>   - Use "cmp r0, #-1", instead of "cmn r0, #1",
>   - Add missing stack setup,
>   - Support appended DTB.
> ---
>  arch/arm/boot/compressed/Makefile            |  6 ++-
>  arch/arm/boot/compressed/fdt_get_mem_start.c | 52 +++++++++++++++++++
>  arch/arm/boot/compressed/head.S              | 54 +++++++++++++++++++-
>  3 files changed, 110 insertions(+), 2 deletions(-)
>  create mode 100644 arch/arm/boot/compressed/fdt_get_mem_start.c
>
> diff --git a/arch/arm/boot/compressed/Makefile b/arch/arm/boot/compressed/Makefile
> index 9c11e7490292f0e0..82e4cee97cb5d905 100644
> --- a/arch/arm/boot/compressed/Makefile
> +++ b/arch/arm/boot/compressed/Makefile
> @@ -86,12 +86,15 @@ libfdt_objs := $(addsuffix .o, $(basename $(libfdt)))
>  $(addprefix $(obj)/,$(libfdt) $(libfdt_hdrs)): $(obj)/%: $(srctree)/scripts/dtc/libfdt/%
>         $(call cmd,shipped)
>
> -$(addprefix $(obj)/,$(libfdt_objs) atags_to_fdt.o): \
> +$(addprefix $(obj)/,$(libfdt_objs) atags_to_fdt.o fdt_get_mem_start.o): \
>         $(addprefix $(obj)/,$(libfdt_hdrs))
>
>  ifeq ($(CONFIG_ARM_ATAG_DTB_COMPAT),y)
>  OBJS   += $(libfdt_objs) atags_to_fdt.o
>  endif
> +ifeq ($(CONFIG_USE_OF),y)
> +OBJS   += $(libfdt_objs) fdt_get_mem_start.o
> +endif
>
>  targets       := vmlinux vmlinux.lds piggy_data piggy.o \
>                  lib1funcs.o ashldi3.o bswapsdi2.o \
> @@ -115,6 +118,7 @@ CFLAGS_fdt.o := $(nossp-flags-y)
>  CFLAGS_fdt_ro.o := $(nossp-flags-y)
>  CFLAGS_fdt_rw.o := $(nossp-flags-y)
>  CFLAGS_fdt_wip.o := $(nossp-flags-y)
> +CFLAGS_fdt_get_mem_start.o := $(nossp-flags-y)
>
>  ccflags-y := -fpic $(call cc-option,-mno-single-pic-base,) -fno-builtin \
>              -I$(obj) $(DISABLE_ARM_SSP_PER_TASK_PLUGIN)
> diff --git a/arch/arm/boot/compressed/fdt_get_mem_start.c b/arch/arm/boot/compressed/fdt_get_mem_start.c
> new file mode 100644
> index 0000000000000000..2c5ac47f656317ee
> --- /dev/null
> +++ b/arch/arm/boot/compressed/fdt_get_mem_start.c
> @@ -0,0 +1,52 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +
> +#include <libfdt.h>
> +
> +static const void *getprop(const void *fdt, const char *node_path,
> +                          const char *property)
> +{
> +       int offset = fdt_path_offset(fdt, node_path);
> +
> +       if (offset == -FDT_ERR_NOTFOUND)
> +               return NULL;
> +
> +       return fdt_getprop(fdt, offset, property, NULL);
> +}
> +
> +static uint32_t get_addr_size(const void *fdt)
> +{
> +       const __be32 *addr_len = getprop(fdt, "/", "#address-cells");
> +
> +       if (!addr_len) {
> +               /* default */
> +               return 1;
> +       }
> +
> +       return fdt32_to_cpu(*addr_len);
> +}
> +
> +/*
> + * Get the start of physical memory
> + */
> +
> +unsigned long fdt_get_mem_start(const void *fdt)
> +{
> +       const __be32 *memory;
> +       uint32_t addr_size;
> +
> +       if (!fdt)
> +               return -1;
> +
> +       if (*(__be32 *)fdt != cpu_to_fdt32(FDT_MAGIC))
> +               return -1;
> +
> +       /* Find the first memory node */
> +       memory = getprop(fdt, "/memory", "reg");
> +       if (!memory)
> +               return -1;
> +
> +       /* There may be multiple cells on LPAE platforms */
> +       addr_size = get_addr_size(fdt);
> +
> +       return fdt32_to_cpu(memory[addr_size - 1]);
> +}
> diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
> index 4f7c6145e31fdc25..e6c06ee19fef2e2d 100644
> --- a/arch/arm/boot/compressed/head.S
> +++ b/arch/arm/boot/compressed/head.S
> @@ -254,8 +254,58 @@ not_angel:
>                 .text
>
>  #ifdef CONFIG_AUTO_ZRELADDR
> +#ifdef CONFIG_USE_OF
>                 /*
> -                * Find the start of physical memory.  As we are executing
> +                * Find the start of physical memory.
> +                * Try the DTB first, if available.
> +                */
> +               adr     r0, LC0
> +               ldr     r1, [r0]        @ get absolute LC0
> +               ldr     sp, [r0, #24]   @ get stack location
> +               sub     r1, r0, r1      @ compute relocation offset
> +               add     sp, sp, r1      @ apply relocation
> +
> +#ifdef CONFIG_ARM_APPENDED_DTB
> +               /*
> +                * Look for an appended DTB. If found, use it and
> +                * move stack away from it.
> +                */
> +               ldr     r6, [r0, #12]   @ get &_edata
> +               add     r6, r6, r1      @ relocate it
> +               ldmia   r6, {r0, r5}    @ get DTB signature and size
> +#ifndef __ARMEB__
> +               ldr     r1, =0xedfe0dd0 @ sig is 0xd00dfeed big endian
> +               /* convert DTB size to little endian */
> +               eor     r2, r5, r5, ror #16
> +               bic     r2, r2, #0x00ff0000
> +               mov     r5, r5, ror #8
> +               eor     r5, r5, r2, lsr #8
> +#else
> +               ldr     r1, =0xd00dfeed
> +#endif
> +               cmp     r0, r1          @ do we have a DTB there?
> +               bne     1f
> +
> +               /* preserve 64-bit alignment */
> +               add     r5, r5, #7
> +               bic     r5, r5, #7
> +               add     sp, sp, r5      @ if so, move stack above DTB
> +               mov     r0, r6          @ and extract memory start from DTB
> +               b       2f
> +
> +1:
> +#endif /* CONFIG_ARM_APPENDED_DTB */
> +
> +               mov     r0, r8
> +2:
> +               bl      fdt_get_mem_start
> +               mov     r4, r0
> +               cmp     r0, #-1
> +               bne     1f
> +#endif /* CONFIG_USE_OF */
> +
> +               /*
> +                * Fall back to the traditional method.  As we are executing
>                  * without the MMU on, we are in the physical address space.
>                  * We just need to get rid of any offset by aligning the
>                  * address.
> @@ -273,6 +323,8 @@ not_angel:
>                  */
>                 mov     r4, pc
>                 and     r4, r4, #0xf8000000
> +
> +1:
>                 /* Determine final kernel image address. */
>                 add     r4, r4, #TEXT_OFFSET
>  #else
> --
> 2.17.1
>

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