At present SPL uses a single stack, either CONFIG_SPL_STACK or
CONFIG_SYS_INIT_SP_ADDR. Since some SPL features (such as MMC and
environment) require a lot of stack, some boards set CONFIG_SPL_STACK to
point into SDRAM. They then set up SDRAM very early, before board_init_f(),
so that the larger stack can be used.

This is an abuse of lowlevel_init(). That function should only be used for
essential start-up code which cannot be delayed. An example of a valid use is
when only part of the SPL code is visible/executable, and the SoC must be set
up so that board_init_f() can be reached. It should not be used for SDRAM
init, console init, etc.

Add a CONFIG_SPL_STACK_R option, which allows the stack to be moved to a new
address before board_init_r() is called in SPL.

The expected SPL flow (for CONFIG_SPL_FRAMEWORK) is now:

Execution starts with start.S. Two main functions can be provided by the
board implementation. The purpose and limitations of each is described below.
After that, the common board_init_r() is called to perform the SPL task.

lowlevel_init():
        - purpose: essential init to permit execution to reach board_init_f()
        - no global_data, but there is a stack
        - must not set up SDRAM or use console
        - must only do the bare minimum to allow execution to continue to
                board_init_f()
        - this is almost never needed
        - return normally from this function

board_init_f():
        - purpose: set up the machine ready for running board_init_r():
                i.e. SDRAM and serial UART
        - global_data is available
        - preloader_console_init() can be called here in extremis
        - stack is in SRAM
        - should set up SDRAM, and anything needed to make the UART work
        - these is no need to clear BSS, it will be done by crt0.S
        - must return normally from this function (don't call board_init_r()
                directly)

Here the BSS is cleared. If CONFIG_SPL_STACK_R is defined, then at this point
the stack and global_data are relocated to below that address.

board_init_r():
        - purpose: main execution, common code
        - global_data is available
        - SDRAM is available
        - stack is optionally in SDRAM, if CONFIG_SPL_STACK_R is defined and
                points into SDRAM
        - preloader_console_init() can be called here - typically this is
                done by defining CONFIG_SPL_BOARD_INIT and then supplying a
                spl_board_init() function containing this call
        - loads U-Boot or (in falcon mode) Linux

Signed-off-by: Simon Glass <s...@chromium.org>
For version 1:
Acked-by: Albert ARIBAUD <albert.u.b...@aribaud.net>
Reviewed-by: Stefan Roese <s...@denx.de>
Tested-by: Bo Shen <voice.s...@atmel.com>
Acked-by: Bo Shen <voice.s...@atmel.com>
Acked-by: Heiko Schocher <h...@denx.de>
Tested-by: Heiko Schocher <h...@denx.de>
---

Changes in v2:
- Move docs to top-level README file and expand them to cover U-Boot proper
- Add Kconfig settings

 Kconfig             | 18 ++++++++++++++
 README              | 68 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 arch/arm/lib/crt0.S | 13 +++++++---
 common/spl/spl.c    | 35 +++++++++++++++++++++++++++
 4 files changed, 131 insertions(+), 3 deletions(-)

diff --git a/Kconfig b/Kconfig
index 4157da3..8398bcd 100644
--- a/Kconfig
+++ b/Kconfig
@@ -92,6 +92,24 @@ config SPL
        help
          If you want to build SPL as well as the normal image, say Y.
 
+config CONFIG_SPL_STACK_R
+       depends on SPL
+       bool "Enable SDRAM location for SPL stack"
+       help
+         SPL starts off execution in SRAM and thus typically has only a small
+         stack available. Since SPL sets up DRAM while in its board_init_f()
+         function, it is possible for the stack to move there before
+         board_init_r() is reached. This option enables a special SDRAM
+         location for the SPL stack. U-Boot SPL switches to this after
+         board_init_f() completes, and before board_init_r() starts.
+
+config CONFIG_SPL_STACK_R_ADDR
+       depends on CONFIG_SPL_STACK_R
+       hex "SDRAM location for SPL stack"
+       help
+         Specify the address in SDRAM for the SPL stack. This will be set up
+         before board_init_r() is called.
+
 config TPL
        bool
        depends on SPL && SUPPORT_TPL
diff --git a/README b/README
index fefa71c..7b48cda 100644
--- a/README
+++ b/README
@@ -273,6 +273,74 @@ run some of U-Boot's tests.
 See board/sandbox/README.sandbox for more details.
 
 
+Board Initialisation Flow:
+--------------------------
+
+This is the intended start-up flow for boards. This should apply for both
+SPL and U-Boot proper (i.e. they both follow the same rules). At present SPL
+mostly uses a separate code path, but the funtion names and roles of each
+function are the same. Some boards or architectures may not conform to this.
+At least most ARM boards which use CONFIG_SPL_FRAMEWORK conform to this.
+
+Execution starts with start.S with three functions called during init after
+that. The purpose and limitations of each is described below.
+
+lowlevel_init():
+       - purpose: essential init to permit execution to reach board_init_f()
+       - no global_data or BSS, but there is a stack
+       - must not set up SDRAM or use console
+       - must only do the bare minimum to allow execution to continue to
+               board_init_f()
+       - this is almost never needed
+       - return normally from this function
+
+board_init_f():
+       - purpose: set up the machine ready for running board_init_r():
+               i.e. SDRAM and serial UART
+       - global_data is available
+       - stack is in SRAM
+       - BSS is not available, so you cannot use global/static variables,
+               only stack variables and global_data
+
+       Non-SPL-specific notes:
+       - dram_init() is called to set up DRAM. If already done in SPL this
+               can do nothing
+
+       SPL-specific notes:
+       - you can override the entire board_init_f() function with your own
+               version as needed.
+       - preloader_console_init() can be called here in extremis
+       - should set up SDRAM, and anything needed to make the UART work
+       - these is no need to clear BSS, it will be done by crt0.S
+       - must return normally from this function (don't call board_init_r()
+               directly)
+
+Here the BSS is cleared. For SPL, if CONFIG_SPL_STACK_R is defined, then at
+this point the stack and global_data are relocated to below
+CONFIG_SPL_STACK_R_ADDR. For non-SPL, U-Boot is relocated to run at the top of
+memory.
+
+board_init_r():
+       - purpose: main execution, common code
+       - global_data is available
+       - SDRAM is available
+       - BSS is available, all static/global variables can be used
+       - execution eventually continues to main_loop()
+
+       Non-SPL-specific notes:
+       - U-Boot is relocated to the top of memory and is now running from
+               there.
+
+       SPL-specific notes:
+       - stack is optionally in SDRAM, if CONFIG_SPL_STACK_R is defined and
+               CONFIG_SPL_STACK_R_ADDR points into SDRAM
+       - preloader_console_init() can be called here - typically this is
+               done by defining CONFIG_SPL_BOARD_INIT and then supplying a
+               spl_board_init() function containing this call
+       - loads U-Boot or (in falcon mode) Linux
+
+
+
 Configuration Options:
 ----------------------
 
diff --git a/arch/arm/lib/crt0.S b/arch/arm/lib/crt0.S
index 22df3e5..7939ced 100644
--- a/arch/arm/lib/crt0.S
+++ b/arch/arm/lib/crt0.S
@@ -113,7 +113,14 @@ here:
 /* Set up final (full) environment */
 
        bl      c_runtime_cpu_setup     /* we still call old routine here */
-
+#endif
+#if !defined(CONFIG_SPL_BUILD) || defined(CONFIG_SPL_FRAMEWORK)
+# ifdef CONFIG_SPL_BUILD
+       /* Use a DRAM stack for the rest of SPL, if requested */
+       bl      spl_relocate_stack_gd
+       cmp     r0, #0
+       movne   sp, r0
+# endif
        ldr     r0, =__bss_start        /* this is auto-relocated! */
        ldr     r1, =__bss_end          /* this is auto-relocated! */
 
@@ -124,9 +131,10 @@ clbss_l:cmp        r0, r1                  /* while not at 
end of BSS */
        addlo   r0, r0, #4              /* move to next */
        blo     clbss_l
 
+#if ! defined(CONFIG_SPL_BUILD)
        bl coloured_LED_init
        bl red_led_on
-
+#endif
        /* call board_init_r(gd_t *id, ulong dest_addr) */
        mov     r0, r9                  /* gd_t */
        ldr     r1, [r9, #GD_RELOCADDR] /* dest_addr */
@@ -134,7 +142,6 @@ clbss_l:cmp r0, r1                  /* while not at end of 
BSS */
        ldr     pc, =board_init_r       /* this is auto-relocated! */
 
        /* we should not return here. */
-
 #endif
 
 ENDPROC(_main)
diff --git a/common/spl/spl.c b/common/spl/spl.c
index 1826c47..78bb279 100644
--- a/common/spl/spl.c
+++ b/common/spl/spl.c
@@ -276,3 +276,38 @@ void preloader_console_init(void)
        spl_display_print();
 #endif
 }
+
+/**
+ * spl_relocate_stack_gd() - Relocate stack ready for board_init_r() execution
+ *
+ * Sometimes board_init_f() runs with a stack in SRAM but we want to use SDRAM
+ * for the main board_init_r() execution. This is typically because we need
+ * more stack space for things like the MMC sub-system.
+ *
+ * This function calculates the stack position, copies the global_data into
+ * place and returns the new stack position. The caller is responsible for
+ * setting up the sp register.
+ *
+ * @return new stack location, or 0 to use the same stack
+ */
+ulong spl_relocate_stack_gd(void)
+{
+#ifdef CONFIG_SPL_STACK_R
+       gd_t *new_gd;
+       ulong ptr;
+
+       /* Get stack position: use 8-byte alignment for ABI compliance */
+       ptr = CONFIG_SPL_STACK_R - sizeof(gd_t);
+       ptr &= ~7;
+       new_gd = (gd_t *)ptr;
+       memcpy(new_gd, (void *)gd, sizeof(gd_t));
+       gd = new_gd;
+
+       /* Clear the BSS. */
+       memset(__bss_start, 0, __bss_end - __bss_start);
+
+       return ptr;
+#else
+       return 0;
+#endif
+}
-- 
2.2.0.rc0.207.ga3a616c

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