riscv: drop the use of XIP_OFFSET in XIP_FIXUP_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_OFFSET. Instead, use CONFIG_PHYS_RAM_BASE and
_sdata to do the same thing.

While at it, also add a description for XIP_FIXUP_OFFSET.

Signed-off-by: Nam Cao <namcao@linutronix.de>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/dba0409518b14ee83b346e099b1f7f934daf7b74.1717789719.git.namcao@linutronix.de
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

diff --git a/arch/riscv/include/asm/xip_fixup.h b/arch/riscv/include/asm/xip_fixup.h
index b65bf6306f69c6f05ba40752d7933bd78382100f..9ed2cfae09e0ee424858878fcf94e7afb7349fa3 100644 (file)
--- a/arch/riscv/include/asm/xip_fixup.h
+++ b/arch/riscv/include/asm/xip_fixup.h
@@ -9,8 +9,19 @@
 
 #ifdef CONFIG_XIP_KERNEL
 .macro XIP_FIXUP_OFFSET reg
-        REG_L t0, _xip_fixup
+       /* Fix-up address in Flash into address in RAM early during boot before
+        * MMU is up. Because generated code "thinks" data is in Flash, but it
+        * is actually in RAM (actually data is also in Flash, but Flash is
+        * read-only, thus we need to use the data residing in RAM).
+        *
+        * The start of data in Flash is _sdata and the start of data in RAM is
+        * CONFIG_PHYS_RAM_BASE. So this fix-up essentially does this:
+        * reg += CONFIG_PHYS_RAM_BASE - _start
+        */
+       li t0, CONFIG_PHYS_RAM_BASE
         add \reg, \reg, t0
+       la t0, _sdata
+       sub \reg, \reg, t0
 .endm
 .macro XIP_FIXUP_FLASH_OFFSET reg
        la t0, __data_loc
@@ -19,7 +30,6 @@
        add \reg, \reg, t0
 .endm
 
-_xip_fixup: .dword CONFIG_PHYS_RAM_BASE - CONFIG_XIP_PHYS_ADDR - XIP_OFFSET
 _xip_phys_offset: .dword CONFIG_XIP_PHYS_ADDR + XIP_OFFSET
 #else
 .macro XIP_FIXUP_OFFSET reg