spl: Allow performing BSS init early before board_init_f()

On some platform we have sufficient memory available early on to allow
setting up and using a basic BSS prior to entering board_init_f(). Doing
so can for example be used to carry state over to board_init_r() without
having to resort to extending U-Boot's global data structure.

To support such scenarios add a Kconfig option called CONFIG_SPL_EARLY_BSS
to allow moving the initialization of BSS prior to entering board_init_f(),
if enabled. Note that using this option usually should go along with using
CONFIG_SPL_SEPARATE_BSS and configuring BSS to be located in memory
actually available prior to board_init_f().

Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>

arch/arm/lib/crt0.S

@@ -57,6 +57,33 @@
  * For more information see 'Board Initialisation Flow in README.
  */
 
+/*
+ * Macro for clearing BSS during SPL execution. Usually called during the
+ * relocation process for most boards before entering board_init_r(), but
+ * can also be done early before entering board_init_f() on plaforms that
+ * can afford it due to sufficient memory being available early.
+ */
+
+.macro SPL_CLEAR_BSS
+	ldr	r0, =__bss_start	/* this is auto-relocated! */
+
+#ifdef CONFIG_USE_ARCH_MEMSET
+	ldr	r3, =__bss_end		/* this is auto-relocated! */
+	mov	r1, #0x00000000		/* prepare zero to clear BSS */
+
+	subs	r2, r3, r0		/* r2 = memset len */
+	bl	memset
+#else
+	ldr	r1, =__bss_end		/* this is auto-relocated! */
+	mov	r2, #0x00000000		/* prepare zero to clear BSS */
+
+clbss_l:cmp	r0, r1			/* while not at end of BSS */
+	strlo	r2, [r0]		/* clear 32-bit BSS word */
+	addlo	r0, r0, #4		/* move to next */
+	blo	clbss_l
+#endif
+.endm
+
 /*
  * entry point of crt0 sequence
  */
@@ -82,6 +109,10 @@ ENTRY(_main)
 	mov	r9, r0
 	bl	board_init_f_init_reserve
 
+#if defined(CONFIG_SPL_EARLY_BSS)
+	SPL_CLEAR_BSS
+#endif
+
 	mov	r0, #0
 	bl	board_init_f
 
@@ -119,6 +150,11 @@ here:
 	bl	c_runtime_cpu_setup	/* we still call old routine here */
 #endif
 #if !defined(CONFIG_SPL_BUILD) || defined(CONFIG_SPL_FRAMEWORK)
+
+#if !defined(CONFIG_SPL_EARLY_BSS)
+	SPL_CLEAR_BSS
+#endif
+
 # ifdef CONFIG_SPL_BUILD
 	/* Use a DRAM stack for the rest of SPL, if requested */
 	bl	spl_relocate_stack_gd
@@ -126,23 +162,6 @@ here:
 	movne	sp, r0
 	movne	r9, r0
 # endif
-	ldr	r0, =__bss_start	/* this is auto-relocated! */
-
-#ifdef CONFIG_USE_ARCH_MEMSET
-	ldr	r3, =__bss_end		/* this is auto-relocated! */
-	mov	r1, #0x00000000		/* prepare zero to clear BSS */
-
-	subs	r2, r3, r0		/* r2 = memset len */
-	bl	memset
-#else
-	ldr	r1, =__bss_end		/* this is auto-relocated! */
-	mov	r2, #0x00000000		/* prepare zero to clear BSS */
-
-clbss_l:cmp	r0, r1			/* while not at end of BSS */
-	strlo	r2, [r0]		/* clear 32-bit BSS word */
-	addlo	r0, r0, #4		/* move to next */
-	blo	clbss_l
-#endif
 
 #if ! defined(CONFIG_SPL_BUILD)
 	bl coloured_LED_init

common/spl/Kconfig

@@ -248,6 +248,16 @@ config TPL_BANNER_PRINT
 	  info. Disabling this option could be useful to reduce SPL boot time
 	  (e.g. approx. 6 ms faster, when output on i.MX6 with 115200 baud).
 
+config SPL_EARLY_BSS
+	depends on ARM && !ARM64
+	bool "Allows initializing BSS early before entering board_init_f"
+	help
+	  On some platform we have sufficient memory available early on to
+	  allow setting up and using a basic BSS prior to entering
+	  board_init_f. Activating this option will also de-activate the
+	  clearing of BSS during the SPL relocation process, thus allowing
+	  to carry state from board_init_f to board_init_r by way of BSS.
+
 config SPL_DISPLAY_PRINT
 	bool "Display a board-specific message in SPL"
 	help