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>

arch/arm/Kconfig

@@ -403,11 +403,29 @@ config ARM_SMCCC
 	  firmware (for example, PSCI) according to SMCCC.
 
 config SEMIHOSTING
-	bool "support boot from semihosting"
+	bool "Support ARM semihosting"
 	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 SPL_SEMIHOSTING
+	bool "Support ARM semihosting in SPL"
+	depends on SPL
+	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 SYS_THUMB_BUILD
 	bool "Build U-Boot using the Thumb instruction set"

arch/arm/include/asm/spl.h

@@ -30,6 +30,7 @@ enum {
 	BOOT_DEVICE_DFU,
 	BOOT_DEVICE_XIP,
 	BOOT_DEVICE_BOOTROM,
+	BOOT_DEVICE_SMH,
 	BOOT_DEVICE_NONE
 };
 #endif

arch/arm/lib/Makefile

@@ -46,7 +46,7 @@ else
 obj-$(CONFIG_$(SPL_TPL_)USE_ARCH_MEMSET) += memset.o
 obj-$(CONFIG_$(SPL_TPL_)USE_ARCH_MEMCPY) += memcpy.o
 endif
-obj-$(CONFIG_SEMIHOSTING) += semihosting.o
+obj-$(CONFIG_$(SPL_TPL_)SEMIHOSTING) += semihosting.o
 
 obj-y	+= bdinfo.o
 obj-y	+= sections.o

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

common/spl/spl_semihosting.c

@@ -0,0 +1,71 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright (C) 2022 Sean Anderson <sean.anderson@seco.com>
+ */
+
+#include <common.h>
+#include <image.h>
+#include <log.h>
+#include <semihosting.h>
+#include <spl.h>
+
+static int smh_read_full(long fd, void *memp, size_t len)
+{
+	long read;
+
+	read = smh_read(fd, memp, len);
+	if (read < 0)
+		return read;
+	if (read != len)
+		return -EIO;
+	return 0;
+}
+
+static int spl_smh_load_image(struct spl_image_info *spl_image,
+			      struct spl_boot_device *bootdev)
+{
+	const char *filename = CONFIG_SPL_FS_LOAD_PAYLOAD_NAME;
+	int ret;
+	long fd, len;
+	struct image_header *header =
+		spl_get_load_buffer(-sizeof(*header), sizeof(*header));
+
+	fd = smh_open(filename, MODE_READ | MODE_BINARY);
+	if (fd < 0) {
+		log_debug("could not open %s: %ld\n", filename, fd);
+		return fd;
+	}
+
+	ret = smh_flen(fd);
+	if (ret < 0) {
+		log_debug("could not get length of image: %d\n", ret);
+		goto out;
+	}
+	len = ret;
+
+	ret = smh_read_full(fd, header, sizeof(struct image_header));
+	if (ret) {
+		log_debug("could not read image header: %d\n", ret);
+		goto out;
+	}
+
+	ret = spl_parse_image_header(spl_image, bootdev, header);
+	if (ret) {
+		log_debug("failed to parse image header: %d\n", ret);
+		goto out;
+	}
+
+	ret = smh_seek(fd, 0);
+	if (ret) {
+		log_debug("could not seek to start of image: %d\n", ret);
+		goto out;
+	}
+
+	ret = smh_read_full(fd, (void *)spl_image->load_addr, len);
+	if (ret)
+		log_debug("could not read %s: %d\n", filename, ret);
+out:
+	smh_close(fd);
+	return ret;
+}
+SPL_LOAD_IMAGE_METHOD("SEMIHOSTING", 0, BOOT_DEVICE_SMH, spl_smh_load_image);