spl: Add semihosting boot method

This adds a boot method for loading the next stage from the host. It is
mostly modeled off of spl_load_image_ext. I am not really sure why/how
spl_load_image_fat uses three different methods to load the image, but
the simple case seems to work OK for now.

To control the presence of this boot method, we add a config symbol.
While we're at it, we update the original semihosting config symbol.

I think semihosting has some advantages of other forms of JTAG boot.
Common other ways to boot from JTAG include:

- Implementing DDR initialization through JTAG (typically with dozens of
  lines of TCL) and then loading U-Boot. The DDR initialization
  typically uses hard-coded register writes, and is not easily adapted
  to different boards. BOOT_DEVICE_SMH allows booting with SPL,
  leveraging U-Boot's existing DDR initialization code. This is the
  method used by NXP's CodeWarrior IDE on Layerscape processors (see
  AN12270).
- Loading a bootloader into SDRAM, waiting for it to initialize DDR, and
  then loading U-Boot. This is tricky, because the debugger must stop the
  boot after the bootloader has completed its work. Trying to load
  U-Boot too early can cause failure to boot. This is the method used by
  Xilinx with its Zynq(MP) processors.
- Loading SPL with BOOT_DEVICE_RAM and breaking before SPL loads the
  image to load U-Boot at the appropriate place. This can be a bit
  tricky, because the load address is dependent on the header size. An
  elf with symbols must also be used in order to stop at the appropriate
  point. BOOT_DEVICE_SMH can be viewed as an extension of this process,
  where SPL automatically stops and tells the host where to place the
  image.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/common/spl/Makefile b/common/spl/Makefile
index db8fd36a261..e71e7bee664 100644
--- a/common/spl/Makefile
+++ b/common/spl/Makefile
@@ -28,6 +28,7 @@ obj-$(CONFIG_$(SPL_TPL_)USB_STORAGE) += spl_usb.o
 obj-$(CONFIG_$(SPL_TPL_)FS_FAT) += spl_fat.o
 obj-$(CONFIG_$(SPL_TPL_)FS_EXT4) += spl_ext.o
 obj-$(CONFIG_$(SPL_TPL_)SATA) += spl_sata.o
+obj-$(CONFIG_$(SPL_TPL_)SEMIHOSTING) += spl_semihosting.o
 obj-$(CONFIG_$(SPL_TPL_)DFU) += spl_dfu.o
 obj-$(CONFIG_$(SPL_TPL_)SPI_LOAD) += spl_spi.o
 obj-$(CONFIG_$(SPL_TPL_)RAM_SUPPORT) += spl_ram.o