[RFC][PATCH 1/3] arm64: Add late jump to kernel board hook

[Marek Vasut]

Add empty weak assembler function armv8_switch_to_el2_prep() which is
jumped to just before U-Boot determines which EL it is running in and
decides which path to take to boot the Linux kernel.

This weak function is meant to be used by architecture specific code
to implement jump to a firmware blob, which then returns right past
this weak function and continues execution of U-Boot code which then
boots the Linux kernel. One example of such use case is when U-Boot
jump tp TFA BL31, which switches from EL3 to EL2 and then returns to
U-Boot code newly running in EL2 and starts the Linux kernel.

The weak function is called with caches already disabled and DM shut
down. Any preparatory work or even loading of more data must be done
in board_prep_linux(), this hook is meant only for the final jump to
the firmware and return to U-Boot before booting Linux.

Signed-off-by: Marek Vasut <marek.vasut+renesas at mailbox.org>
---
Cc: Andre Przywara <andre.przywara at arm.com>
Cc: Caleb Connolly <caleb.connolly at linaro.org>
Cc: Igor Opaniuk <igor.opaniuk at gmail.com>
Cc: Ilias Apalodimas <ilias.apalodimas at linaro.org>
Cc: Julien Masson <jmasson at baylibre.com>
Cc: Mattijs Korpershoek <mkorpershoek at baylibre.com>
Cc: Maxim Moskalets <maximmosk4 at gmail.com>
Cc: Michael Walle <mwalle at kernel.org>
Cc: Nobuhiro Iwamatsu <iwamatsu at nigauri.org>
Cc: Patrick Rudolph <patrick.rudolph at 9elements.com>
Cc: Paul Barker <paul.barker.ct at bp.renesas.com>
Cc: Paul-Erwan Rio <paulerwan.rio at gmail.com>
Cc: Peter Hoyes <Peter.Hoyes at arm.com>
Cc: Raymond Mao <raymond.mao at linaro.org>
Cc: Sam Protsenko <semen.protsenko at linaro.org>
Cc: Simon Glass <sjg at chromium.org>
Cc: Sughosh Ganu <sughosh.ganu at linaro.org>
Cc: Tom Rini <trini at konsulko.com>
Cc: u-boot at lists.denx.de
---
 arch/arm/cpu/armv8/transition.S |  8 ++++++++
 arch/arm/include/asm/system.h   | 16 ++++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/arch/arm/cpu/armv8/transition.S b/arch/arm/cpu/armv8/transition.S
index 9dbdff3a4fc..85f13ccd0d2 100644
--- a/arch/arm/cpu/armv8/transition.S
+++ b/arch/arm/cpu/armv8/transition.S
@@ -9,8 +9,16 @@
 #include <linux/linkage.h>
 #include <asm/macro.h>
 
+.pushsection .text.armv8_switch_to_el2_prep, "ax"
+WEAK(armv8_switch_to_el2_prep)
+	ret
+ENDPROC(armv8_switch_to_el2_prep)
+.popsection
+
 .pushsection .text.armv8_switch_to_el2, "ax"
 ENTRY(armv8_switch_to_el2)
+	bl	armv8_switch_to_el2_prep
+	nop
 	switch_el x6, 1f, 0f, 0f
 0:
 	cmp x5, #ES_TO_AARCH64
diff --git a/arch/arm/include/asm/system.h b/arch/arm/include/asm/system.h
index dbf9ab43e28..091082281c7 100644
--- a/arch/arm/include/asm/system.h
+++ b/arch/arm/include/asm/system.h
@@ -238,6 +238,22 @@ int __asm_flush_l3_dcache(void);
 int __asm_invalidate_l3_icache(void);
 void __asm_switch_ttbr(u64 new_ttbr);
 
+/*
+ * armv8_switch_to_el2_prep() - prepare for switch from EL3 to EL2 for ARMv8
+ *
+ * @args:        For loading 64-bit OS, fdt address.
+ *               For loading 32-bit OS, zero.
+ * @mach_nr:     For loading 64-bit OS, zero.
+ *               For loading 32-bit OS, machine nr
+ * @fdt_addr:    For loading 64-bit OS, zero.
+ *               For loading 32-bit OS, fdt address.
+ * @arg4:	 Input argument.
+ * @entry_point: kernel entry point
+ * @es_flag:     execution state flag, ES_TO_AARCH64 or ES_TO_AARCH32
+ */
+void armv8_switch_to_el2_prep(u64 args, u64 mach_nr, u64 fdt_addr,
+			      u64 arg4, u64 entry_point, u64 es_flag);
+
 /*
  * armv8_switch_to_el2() - switch from EL3 to EL2 for ARMv8
  *
-- 
2.45.2