common: Rewrite hiding the end of memory

As the name may be confusing, the CONFIG_SYS_MEM_TOP_HIDE reserves
some memory from the end of ram, tracked by gd->ram_size. It is not
always the top of u-boot visible memory. Rewrite the macro with a
weak function to provide flexibility for complex calcuation. Legacy
use of this macro is still supported.

Signed-off-by: York Sun <yorksun@freescale.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

README

@@ -3877,7 +3877,7 @@ Configuration Settings:
 		the RAM base is not zero, or RAM is divided into banks,
 		this variable needs to be recalcuated to get the address.
 
-- CONFIG_SYS_MEM_TOP_HIDE (PPC only):
+- CONFIG_SYS_MEM_TOP_HIDE:
 		If CONFIG_SYS_MEM_TOP_HIDE is defined in the board config header,
 		this specified memory area will get subtracted from the top
 		(end) of RAM and won't get "touched" at all by U-Boot. By

common/board_f.c

@@ -317,6 +317,15 @@ __weak ulong board_get_usable_ram_top(ulong total_size)
 	return gd->ram_top;
 }
 
+__weak phys_size_t board_reserve_ram_top(phys_size_t ram_size)
+{
+#ifdef CONFIG_SYS_MEM_TOP_HIDE
+	return ram_size - CONFIG_SYS_MEM_TOP_HIDE;
+#else
+	return ram_size;
+#endif
+}
+
 static int setup_dest_addr(void)
 {
 	debug("Monitor len: %08lX\n", gd->mon_len);
@@ -333,19 +342,17 @@ static int setup_dest_addr(void)
 	 */
 	gd->secure_ram = gd->ram_size;
 #endif
-#if defined(CONFIG_SYS_MEM_TOP_HIDE)
 	/*
 	 * Subtract specified amount of memory to hide so that it won't
 	 * get "touched" at all by U-Boot. By fixing up gd->ram_size
 	 * the Linux kernel should now get passed the now "corrected"
-	 * memory size and won't touch it either. This should work
+	 * memory size and won't touch it either. This has been used
-	 * for arch/ppc and arch/powerpc. Only Linux board ports in
+	 * by arch/powerpc exclusively. Now ARMv8 takes advantage of
-	 * arch/powerpc with bootwrapper support, that recalculate the
+	 * thie mechanism. If memory is split into banks, addresses
-	 * memory size from the SDRAM controller setup will have to
+	 * need to be calculated.
-	 * get fixed.
 	 */
-	gd->ram_size -= CONFIG_SYS_MEM_TOP_HIDE;
+	gd->ram_size = board_reserve_ram_top(gd->ram_size);
-#endif
+
 #ifdef CONFIG_SYS_SDRAM_BASE
 	gd->ram_top = CONFIG_SYS_SDRAM_BASE;
 #endif