armv8: make SPL exception vectors optional

Even though the exception vector table is a fundamental part of the ARM
architecture, U-Boot mostly does not make real use of it, except when
crash dumping. But having it in takes up quite some space, partly due to
the architectural alignment requirement of 2KB. Since we don't take special
care of that, the compiler adds a more or less random amount of padding
space, which increases the image size quite a bit, especially for the SPL.

On a typical Allwinner build this is around 1.5KB of padding, plus 1KB
for the vector table (mostly padding space again), then some extra code
to do the actual handling. This amounts to almost 10% of the maximum image
size, which is quite a lot for a pure debugging feature.

Add a Kconfig symbol to allow the exception vector table to be left out
of the build for the SPL.
For now this is "default y" for everyone, but specific defconfigs,
platforms or .config files can opt out here at will, to mitigate the code
size pressure we see for some SPLs.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>

arch/arm/cpu/armv8/Kconfig

@@ -1,5 +1,16 @@
 if ARM64
 
+config ARMV8_SPL_EXCEPTION_VECTORS
+	bool "Install crash dump exception vectors"
+	depends on SPL
+	default y
+	help
+	  The default exception vector table is only used for the crash
+	  dump, but still takes quite a lot of space in the image size.
+
+	  Say N here if you are running out of code space in the image
+	  and want to save some space at the cost of less debugging info.
+
 config ARMV8_MULTIENTRY
         bool "Enable multiple CPUs to enter into U-Boot"
 

arch/arm/cpu/armv8/Makefile

@@ -10,7 +10,11 @@ ifndef CONFIG_$(SPL_TPL_)TIMER
 obj-$(CONFIG_SYS_ARCH_TIMER) += generic_timer.o
 endif
 obj-y	+= cache_v8.o
+ifdef CONFIG_SPL_BUILD
+obj-$(CONFIG_ARMV8_SPL_EXCEPTION_VECTORS) += exceptions.o
+else
 obj-y	+= exceptions.o
+endif
 obj-y	+= cache.o
 obj-y	+= tlb.o
 obj-y	+= transition.o

arch/arm/cpu/armv8/start.S

@@ -86,14 +86,23 @@ pie_fixup_done:
 
 #ifdef CONFIG_SYS_RESET_SCTRL
 	bl reset_sctrl
+#endif
+
+#if defined(CONFIG_ARMV8__SPL_EXCEPTION_VECTORS) || !defined(CONFIG_SPL_BUILD)
+.macro	set_vbar, regname, reg
+	msr	\regname, \reg
+.endm
+	adr	x0, vectors
+#else
+.macro	set_vbar, regname, reg
+.endm
 #endif
 	/*
 	 * Could be EL3/EL2/EL1, Initial State:
 	 * Little Endian, MMU Disabled, i/dCache Disabled
 	 */
-	adr	x0, vectors
 	switch_el x1, 3f, 2f, 1f
-3:	msr	vbar_el3, x0
+3:	set_vbar vbar_el3, x0
 	mrs	x0, scr_el3
 	orr	x0, x0, #0xf			/* SCR_EL3.NS|IRQ|FIQ|EA */
 	msr	scr_el3, x0
@@ -103,11 +112,11 @@ pie_fixup_done:
 	msr	cntfrq_el0, x0			/* Initialize CNTFRQ */
 #endif
 	b	0f
-2:	msr	vbar_el2, x0
+2:	set_vbar	vbar_el2, x0
 	mov	x0, #0x33ff
 	msr	cptr_el2, x0			/* Enable FP/SIMD */
 	b	0f
-1:	msr	vbar_el1, x0
+1:	set_vbar	vbar_el1, x0
 	mov	x0, #3 << 20
 	msr	cpacr_el1, x0			/* Enable FP/SIMD */
 0:
@@ -345,6 +354,7 @@ ENDPROC(smp_kick_all_cpus)
 /*-----------------------------------------------------------------------*/
 
 ENTRY(c_runtime_cpu_setup)
+#if defined(CONFIG_ARMV8__SPL_EXCEPTION_VECTORS) || !defined(CONFIG_SPL_BUILD)
 	/* Relocate vBAR */
 	adr	x0, vectors
 	switch_el x1, 3f, 2f, 1f
@@ -354,6 +364,7 @@ ENTRY(c_runtime_cpu_setup)
 	b	0f
 1:	msr	vbar_el1, x0
 0:
+#endif
 
 	ret
 ENDPROC(c_runtime_cpu_setup)