arm: Allow skipping of low-level init with I-cache on

At present CONFIG_SKIP_LOWLEVEL_INIT prevents U-Boot from calling
lowlevel_init(). This means that the instruction cache is not enabled and
the board runs very slowly.

What is really needed in many cases is to skip the call to lowlevel_init()
but still perform CP15 init. Add an option to handle this.

Reviewed-by: Heiko Schocher <hs@denx.de>
Tested-on: smartweb, corvus, taurus, axm
Tested-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Andreas Bießmann <andreas@biessmann.org>

README

@@ -4824,6 +4824,11 @@ Low Level (hardware related) configuration options:
 		other boot loader or by a debugger which performs
 		these initializations itself.
 
+- CONFIG_SKIP_LOWLEVEL_INIT_ONLY
+		[ARM926EJ-S only] This allows just the call to lowlevel_init()
+		to be skipped. The normal CPU15 init (such as enabling the
+		instruction cache) is still performed.
+
 - CONFIG_SPL_BUILD
 		Modifies the behaviour of start.S when compiling a loader
 		that is executed before the actual U-Boot. E.g. when

arch/arm/cpu/arm1136/start.S

@@ -82,6 +82,7 @@ cpu_init_crit:
 	orr	r0, r0, #0x00001000	@ set bit 12 (I) I-Cache
 	mcr	p15, 0, r0, c1, c0, 0
 
+#ifndef CONFIG_SKIP_LOWLEVEL_INIT_ONLY
 	/*
 	 * Jump to board specific initialization... The Mask ROM will have already initialized
 	 * basic memory.  Go here to bump up clock rate and handle wake up conditions.
@@ -89,5 +90,6 @@ cpu_init_crit:
 	mov	ip, lr		/* persevere link reg across call */
 	bl	lowlevel_init	/* go setup pll,mux,memory */
 	mov	lr, ip		/* restore link */
+#endif
 	mov	pc, lr		/* back to my caller */
 #endif /* CONFIG_SKIP_LOWLEVEL_INIT */

arch/arm/cpu/arm920t/start.S

@@ -135,6 +135,7 @@ cpu_init_crit:
 	orr	r0, r0, #0x00001000	@ set bit 12 (I) I-Cache
 	mcr	p15, 0, r0, c1, c0, 0
 
+#ifndef CONFIG_SKIP_LOWLEVEL_INIT_ONLY
 	/*
 	 * before relocating, we have to setup RAM timing
 	 * because memory timing is board-dependend, you will
@@ -143,7 +144,7 @@ cpu_init_crit:
 	mov	ip, lr
 
 	bl	lowlevel_init
-
 	mov	lr, ip
+#endif
 	mov	pc, lr
 #endif /* CONFIG_SKIP_LOWLEVEL_INIT */

arch/arm/cpu/arm926ejs/start.S

@@ -101,11 +101,13 @@ flush_dcache:
 #endif
 	mcr	p15, 0, r0, c1, c0, 0
 
+#ifndef CONFIG_SKIP_LOWLEVEL_INIT_ONLY
 	/*
 	 * Go setup Memory and board specific bits prior to relocation.
 	 */
 	mov	ip, lr		/* perserve link reg across call */
 	bl	lowlevel_init	/* go setup pll,mux,memory */
 	mov	lr, ip		/* restore link */
+#endif
 	mov	pc, lr		/* back to my caller */
 #endif /* CONFIG_SKIP_LOWLEVEL_INIT */

arch/arm/cpu/arm946es/start.S

@@ -90,11 +90,13 @@ cpu_init_crit:
 	orr	r0, r0, #0x00001000	/* set bit 12 (I) I-Cache */
 	mcr	p15, 0, r0, c1, c0, 0
 
+#ifndef CONFIG_SKIP_LOWLEVEL_INIT_ONLY
 	/*
 	 * Go setup Memory and board specific bits prior to relocation.
 	 */
 	mov	ip, lr		/* perserve link reg across call */
 	bl	lowlevel_init	/* go setup memory */
 	mov	lr, ip		/* restore link */
+#endif
 	mov	pc, lr		/* back to my caller */
 #endif

arch/arm/cpu/armv7/start.S

@@ -66,7 +66,9 @@ save_boot_params_ret:
 	/* the mask ROM code should have PLL and others stable */
 #ifndef CONFIG_SKIP_LOWLEVEL_INIT
 	bl	cpu_init_cp15
+#ifndef CONFIG_SKIP_LOWLEVEL_INIT_ONLY
 	bl	cpu_init_crit
+#endif
 #endif
 
 	bl	_main
@@ -250,7 +252,8 @@ skip_errata_621766:
 	mov	pc, r5			@ back to my caller
 ENDPROC(cpu_init_cp15)
 
-#ifndef CONFIG_SKIP_LOWLEVEL_INIT
+#if !defined(CONFIG_SKIP_LOWLEVEL_INIT) && \
+	!defined(CONFIG_SKIP_LOWLEVEL_INIT_ONLY)
 /*************************************************************************
  *
  * CPU_init_critical registers

arch/arm/cpu/sa1100/start.S

@@ -96,6 +96,7 @@ cpu_init_crit:
 	ldr	r1, cpuspeed
 	str	r1, [r0, #PPCR]
 
+#ifndef CONFIG_SKIP_LOWLEVEL_INIT_ONLY
 	/*
 	 * before relocating, we have to setup RAM timing
 	 * because memory timing is board-dependend, you will
@@ -104,6 +105,7 @@ cpu_init_crit:
 	mov	ip,	lr
 	bl	lowlevel_init
 	mov	lr,	ip
+#endif
 
 	/*
 	 * disable MMU stuff and enable I-cache