Big endian modifications

[Rory Bolt]

Hello,

I have been modifying GRUB on Das U-boot to allow it to boot aarch64_be big endian kernels on the RockPro64 board. Since the RockPro64 has a PCIe (albeit 2.0) slot, it provides a low cost/reasonable performance (vs QEMU) system to test our devices, drivers and software stack on a big endian system. I am in the process of publishing the required changes to GRUB, however there are two minor changes made to the kernel itself. Arguably the modification to head.S could be moved to grub itself, however it is in the defensive spirit of not assuming the boot loader has set up everything correctly and is very similar to the startup of NetBSD which contains similar code in its startup. The second change to Kconfig allows a EFI/PE/COFF header to be added to a big endian kernel (although unfortunately it also adds a useless big endian efistub to the kernel too). With the following two changes the modified GRUB can boot either big endian or little endian Arm64 images.

Please consider the following two modifications for inclusion:

diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index 6a98f1a38c29..40a18d767d15 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -89,6 +89,24 @@
         *  x24        __primary_switch() .. relocate_kernel()  current RELR displacement
         */
 SYM_CODE_START(primary_entry)
+#ifdef CONFIG_CPU_BIG_ENDIAN
+        mrs     x21, CurrentEL
+        lsr     x21, x21, #2
+        cmp     x21, #0x2
+        b.lo    1f
+
+        mrs     x21, sctlr_el2
+        orr     x21, x21, #SCTLR_ELx_EE /* set: Big Endian */
+        msr     sctlr_el2, x21
+        isb
+
+1:
+        mrs     x21, sctlr_el1
+        orr     x21, x21, #(SCTLR_ELx_EE | SCTLR_EL1_E0E)       /* set: Big Endian */
+        msr     sctlr_el1, x21
+        isb
+
+#endif
        bl      preserve_boot_args
        bl      init_kernel_el                  // w0=cpu_boot_mode
        adrp    x23, __PHYS_OFFSET


diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index c4207cf9bb17..a9ccbeb75ea7 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1997,7 +1997,7 @@ config EFI_STUB

 config EFI
        bool "UEFI runtime support"
-       depends on OF && !CPU_BIG_ENDIAN
+       depends on OF
        depends on KERNEL_MODE_NEON
        select ARCH_SUPPORTS_ACPI
        select LIBFDT

Rory Bolt

KIOXIA America, Inc. | formerly Toshiba Memory America, Inc.