Support --without-payload for OpenSBI fw_jump-style booting (#186)

We expect the firmware to load the external payload at the second
megapage, and that there is space to put the filtered FDT at 0x2200000
past the start of memory. With a default MEM_START of 0x80000000, this
matches the standard OpenSBI values for FW_JUMP_ADDR and
FW_JUMP_FDT_ADDR of 0x80400000/0x80200000 (RV32/RV64) and 0x82200000
respectively, so payloads linked for one should work with the other.

Makefile.in

@@ -111,7 +111,10 @@ VPATH := $(addprefix $(src_dir)/, $(sprojs_enabled))
 CC            := @CC@
 READELF       := @READELF@
 OBJCOPY       := @OBJCOPY@
-CFLAGS        := @CFLAGS@ $(CFLAGS) $(march) $(mabi) -DBBL_PAYLOAD=\"bbl_payload\" -DBBL_LOGO_FILE=\"bbl_logo_file\" -fno-stack-protector -U_FORTIFY_SOURCE
+CFLAGS        := @CFLAGS@ $(CFLAGS) $(march) $(mabi) -DBBL_LOGO_FILE=\"bbl_logo_file\" -DMEM_START=@MEM_START@ -fno-stack-protector -U_FORTIFY_SOURCE
+ifneq (@BBL_PAYLOAD@,no)
+CFLAGS        := $(CFLAGS) -DBBL_PAYLOAD=\"bbl_payload\"
+endif
 BBL_PAYLOAD   := @BBL_PAYLOAD@
 COMPILE       := $(CC) -MMD -MP $(CFLAGS) \
                  $(sprojs_include)

bbl/bbl.c

@@ -9,7 +9,14 @@
 #include "fdt.h"
 #include <string.h>
 
+#ifdef BBL_PAYLOAD
 extern char _payload_start, _payload_end; /* internal payload */
+# define PAYLOAD_START &_payload_start
+# define PAYLOAD_END ROUNDUP((uintptr_t)&_payload_end, MEGAPAGE_SIZE)
+#else
+# define PAYLOAD_START (void*)(MEM_START + MEGAPAGE_SIZE)
+# define PAYLOAD_END (void*)(MEM_START + 0x2200000)
+#endif
 static const void* entry_point;
 long disabled_hart_mask;
 
@@ -23,8 +30,9 @@ static uintptr_t dtb_output()
    * address. The kernel's virtual mapping begins at its load address,
    * thus mandating device-tree is in physical memory after the kernel.
    */
-  uintptr_t end = kernel_end ? (uintptr_t)kernel_end : (uintptr_t)&_payload_end;
-  return (end + MEGAPAGE_SIZE - 1) / MEGAPAGE_SIZE * MEGAPAGE_SIZE;
+  uintptr_t end = kernel_end ? ROUNDUP((uintptr_t)kernel_end, MEGAPAGE_SIZE)
+                             : (uintptr_t)PAYLOAD_END;
+  return end;
 }
 
 static void filter_dtb(uintptr_t source)
@@ -123,6 +131,6 @@ void boot_loader(uintptr_t dtb)
 #endif
   mb();
   /* Use optional FDT preloaded external payload if present */
-  entry_point = kernel_start ? kernel_start : &_payload_start;
+  entry_point = kernel_start ? kernel_start : PAYLOAD_START;
   boot_other_hart(0);
 }

bbl/bbl.mk.in

@@ -4,7 +4,6 @@ bbl_subproject_deps = \
 	util \
   softfloat \
   machine \
-  dummy_payload \
 
 bbl_hdrs = \
   bbl.h \
@@ -13,13 +12,22 @@ bbl_c_srcs = \
   logo.c \
 
 bbl_asm_srcs = \
-  payload.S \
   raw_logo.S \
 
+ifeq (@BBL_PAYLOAD@,dummy_payload)
+bbl_subproject_deps += \
+  dummy_payload
+endif
+
+ifneq (@BBL_PAYLOAD@,no)
+bbl_asm_srcs += \
+  payload.S
+
 payload.o: bbl_payload
 
 bbl_payload: $(BBL_PAYLOAD)
 	if $(READELF) -h $< 2> /dev/null > /dev/null; then $(OBJCOPY) -O binary --set-section-flags .bss=alloc,load,contents $< $@; else cp $< $@; fi
+endif
 
 raw_logo.o: bbl_logo_file