Makefile: ensure DTB doesn't overflow into initial stack

With CONFIG_SYS_INIT_SP_BSS_OFFSET enabled, the initial (pre-relocation)
stack is placed some distance after bss_start. The control DTB is appended
to the U-Boot binary at bss_start. If the DTB is too large, or the SP BSS
offset too small, then the initial stack could corrupt the DTB. Enhance
the Makefile to check whether this is likely to occur.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>

Makefile

@@ -811,6 +811,10 @@ ifneq ($(CONFIG_BUILD_TARGET),)
 ALL-y += $(CONFIG_BUILD_TARGET:"%"=%)
 endif
 
+ifneq ($(CONFIG_SYS_INIT_SP_BSS_OFFSET),)
+ALL-y += init_sp_bss_offset_check
+endif
+
 LDFLAGS_u-boot += $(LDFLAGS_FINAL)
 
 # Avoid 'Not enough room for program headers' error on binutils 2.28 onwards.
@@ -939,6 +943,33 @@ binary_size_check: u-boot-nodtb.bin FORCE
 		fi \
 	fi
 
+ifneq ($(CONFIG_SYS_INIT_SP_BSS_OFFSET),)
+ifneq ($(CONFIG_SYS_MALLOC_F_LEN),)
+subtract_sys_malloc_f_len = space=$$(($${space} - $(CONFIG_SYS_MALLOC_F_LEN)))
+else
+subtract_sys_malloc_f_len = true
+endif
+# The 1/4 margin below is somewhat arbitrary. The likely initial SP usage is
+# so low that the DTB could probably use 90%+ of the available space, for
+# current values of CONFIG_SYS_INIT_SP_BSS_OFFSET at least. However, let's be
+# safe for now and tweak this later if space becomes tight.
+# A rejected alternative would be to check that some absolute minimum stack
+# space was available. However, since CONFIG_SYS_INIT_SP_BSS_OFFSET is
+# deliberately build-specific, to take account of build-to-build stack usage
+# differences due to different feature sets, there is no common absolute value
+# to check against.
+init_sp_bss_offset_check: u-boot.dtb FORCE
+	@dtb_size=$(shell wc -c u-boot.dtb | awk '{print $$1}') ; \
+	space=$(CONFIG_SYS_INIT_SP_BSS_OFFSET) ; \
+	$(subtract_sys_malloc_f_len) ; \
+	quarter_space=$$(($${space} / 4)) ; \
+	if [ $${dtb_size} -gt $${quarter_space} ]; then \
+		echo "u-boot.dtb is larger than 1 quarter of " >&2 ; \
+		echo "(CONFIG_SYS_INIT_SP_BSS_OFFSET - CONFIG_SYS_MALLOC_F_LEN)" >&2 ; \
+		exit 1 ; \
+	fi
+endif
+
 u-boot-nodtb.bin: u-boot FORCE
 	$(call if_changed,objcopy)
 	$(call DO_STATIC_RELA,$<,$@,$(CONFIG_SYS_TEXT_BASE))