2024-05-10 06:29:09

by Nam Cao

Subject: [PATCH 4/7] riscv: drop the use of XIP_OFFSET in XIP_FIXUP_FLASH_OFFSET

XIP_OFFSET is the hard-coded offset of writable data section within the
kernel.

By hard-coding this value, the read-only section of the kernel (which is
placed before the writable data section) is restricted in size.

As a preparation to remove this hard-coded macro XIP_OFFSET entirely, stop
using XIP_OFFSET in XIP_FIXUP_FLASH_OFFSET. Instead, use __data_loc and
_sdata to do the same thing.

While at it, also add a description for XIP_FIXUP_FLASH_OFFSET.

Signed-off-by: Nam Cao <[email protected]>
---
arch/riscv/include/asm/xip_fixup.h | 16 ++++++++++++----
1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/arch/riscv/include/asm/xip_fixup.h b/arch/riscv/include/asm/xip_fixup.h
index 9ed2cfae09e0..f3d56299bc22 100644
--- a/arch/riscv/include/asm/xip_fixup.h
+++ b/arch/riscv/include/asm/xip_fixup.h
@@ -24,13 +24,21 @@
sub \reg, \reg, t0
.endm
.macro XIP_FIXUP_FLASH_OFFSET reg
+ /* In linker script, at the transition from read-only section to
+ * writable section, the VMA is increased while LMA remains the same.
+ * (See in linker script how _sdata, __data_loc and LOAD_OFFSET is
+ * changed)
+ *
+ * Consequently, early during boot before MMU is up, the generated code
+ * reads the "writable" section at wrong addresses, because VMA is used
+ * by compiler to generate code, but the data is located in Flash using
+ * LMA.
+ */
+ la t0, _sdata
+ sub \reg, \reg, t0
la t0, __data_loc
- REG_L t1, _xip_phys_offset
- sub \reg, \reg, t1
add \reg, \reg, t0
.endm
-
-_xip_phys_offset: .dword CONFIG_XIP_PHYS_ADDR + XIP_OFFSET
#else
.macro XIP_FIXUP_OFFSET reg
.endm
--
2.39.2

2024-05-27 12:47:16

by Alexandre Ghiti

Subject: Re: [PATCH 4/7] riscv: drop the use of XIP_OFFSET in XIP_FIXUP_FLASH_OFFSET

On 10/05/2024 08:28, Nam Cao wrote:
> XIP_OFFSET is the hard-coded offset of writable data section within the
> kernel.
>
> By hard-coding this value, the read-only section of the kernel (which is
> placed before the writable data section) is restricted in size.
>
> As a preparation to remove this hard-coded macro XIP_OFFSET entirely, stop
> using XIP_OFFSET in XIP_FIXUP_FLASH_OFFSET. Instead, use __data_loc and
> _sdata to do the same thing.
>
> While at it, also add a description for XIP_FIXUP_FLASH_OFFSET.
>
> Signed-off-by: Nam Cao <[email protected]>
> ---
> arch/riscv/include/asm/xip_fixup.h | 16 ++++++++++++----
> 1 file changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/arch/riscv/include/asm/xip_fixup.h b/arch/riscv/include/asm/xip_fixup.h
> index 9ed2cfae09e0..f3d56299bc22 100644
> --- a/arch/riscv/include/asm/xip_fixup.h
> +++ b/arch/riscv/include/asm/xip_fixup.h
> @@ -24,13 +24,21 @@
> sub \reg, \reg, t0
> .endm
> .macro XIP_FIXUP_FLASH_OFFSET reg
> + /* In linker script, at the transition from read-only section to
> + * writable section, the VMA is increased while LMA remains the same.
> + * (See in linker script how _sdata, __data_loc and LOAD_OFFSET is
> + * changed)
> + *
> + * Consequently, early during boot before MMU is up, the generated code
> + * reads the "writable" section at wrong addresses, because VMA is used
> + * by compiler to generate code, but the data is located in Flash using
> + * LMA.
> + */
> + la t0, _sdata
> + sub \reg, \reg, t0
> la t0, __data_loc
> - REG_L t1, _xip_phys_offset
> - sub \reg, \reg, t1
> add \reg, \reg, t0
> .endm
> -
> -_xip_phys_offset: .dword CONFIG_XIP_PHYS_ADDR + XIP_OFFSET
> #else
> .macro XIP_FIXUP_OFFSET reg
> .endm


Reviewed-by: Alexandre Ghiti <[email protected]>

Thanks,

Alex