[PATCH v3 11/29] spl: Add semihosting boot method

[Tom Rini]

On Tue, Mar 22, 2022 at 04:59:19PM -0400, Sean Anderson wrote:

> 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 at seco.com>

Applied to u-boot/next, thanks!

-- 
Tom
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