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>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 1f2b849..b2e2873 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -403,11 +403,29 @@
firmware (for example, PSCI) according to SMCCC.
config SEMIHOSTING
- bool "support boot from semihosting"
+ bool "Support ARM semihosting"
+ help
+ Semihosting is a method for a target to communicate with a host
+ debugger. It uses special instructions which the debugger will trap
+ on and interpret. This allows U-Boot to read/write files, print to
+ the console, and execute arbitrary commands on the host system.
+
+ Enabling this option will add support for reading and writing files
+ on the host system. If you don't have a debugger attached then trying
+ to do this will likely cause U-Boot to hang. Say 'n' if you are unsure.
+
+config SPL_SEMIHOSTING
+ bool "Support ARM semihosting in SPL"
+ depends on SPL
help
- In emulated environments, semihosting is a way for
- the hosted environment to call out to the emulator to
- retrieve files from the host machine.
+ Semihosting is a method for a target to communicate with a host
+ debugger. It uses special instructions which the debugger will trap
+ on and interpret. This allows U-Boot to read/write files, print to
+ the console, and execute arbitrary commands on the host system.
+
+ Enabling this option will add support for reading and writing files
+ on the host system. If you don't have a debugger attached then trying
+ to do this will likely cause U-Boot to hang. Say 'n' if you are unsure.
config SYS_THUMB_BUILD
bool "Build U-Boot using the Thumb instruction set"