Add uboot instructions

docs/uboot_raspi3.md

@@ -0,0 +1,18 @@
+How to use u-boot to boot rCore in Raspberry Pi
+===============
+
+Tested under QEMU.
+
+Instructions:
+
+1. Build u-boot
+   1. Download aarch64 toolchain and u-boot source
+   2. `make rpi_3_defconfig ARCH=arm CROSS_COMPILE=aarch64-elf-`
+   3. `make all ARCH=arm CROSS_COMILE=aarch64-elf-`
+   4. A file named `u-boot.bin` should be generated
+2. Use u-boot to run rCore
+   1. `make run arch=aarch64 u_boot=/path/to/u-boot.bin`
+   2. In u-boot, enter following commands:
+      1. `mmc read 0x1000000 0 ${nblocks}`, where ${nblocks} can be probed if you enter a large enought number
+      2. `bootelf -p 0x1000000`
+   3. rCore should boot now

kernel/Makefile

@@ -21,13 +21,14 @@
 #   LOG  = off | error | warn | info | debug | trace
 #   SFSIMG = <sfsimg>            SFS image path of user programs
 #   smp     = 1 | 2 | ...        SMP core number
-#   graphic = on | off              enable/disable qemu graphical output
+#   graphic = on | off           Enable/disable qemu graphical output
 #   board   = none               Running on QEMU
 #         | u540                 Only available on riscv64, run on HiFive U540, use Sv39
 #         | raspi3               Only available on aarch64, run on Raspberry Pi 3 Model B/B+
 #   pci_passthru = 0000:00:00.1  Only available on x86_64, passthrough the specified PCI device
 #   init = /bin/ls               Only available on riscv64, run specified program instead of user shell
 #   extra_nic = on | off         Only available on x86_64, add an additional e1000 nic
+#   u_boot = /path/to/u-boot.bin Only available on aarch64, use u-boot to boot rcore
 
 arch ?= riscv64
 board ?= none
@@ -114,6 +115,10 @@ qemu_opts += \
 	-machine $(board) \
 	-serial null -serial mon:stdio \
 	-kernel $(kernel_img)
+ifneq ($(u_boot), )
+qemu_opts += \
+	-sd $(bootloader)
+endif
 endif
 
 ifdef d
@@ -262,8 +267,12 @@ else ifeq ($(arch), riscv64)
 	make -j && \
 	cp bbl $(abspath $@)
 else ifeq ($(arch), aarch64)
+ifneq ($(u_boot), )
+	@cp $(u_boot) $@
+else
 	@$(objcopy) $(bootloader) --strip-all -O binary $@
 endif
+endif
 
 kernel:
 	@echo Building $(arch) kernel