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echoplayer: implement boot from debugger

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Add a 'make start' target which starts Rockbox using a
debugger. This only works to load the main binary, but
makes it much faster to test changes that don't affect
plugins/codecs.

Because SDRAM isn't accessible at reset and the main
binary is usually too big to fit in SRAM, the bootloader
must be flashed first before Rockbox can be loaded in
this way.

The boot protocol involves GDB writing a check pattern
to SRAM while the CPU is held in reset. The bootloader
detects the pattern and takes a breakpoint, by which
time SDRAM is accessible; GDB can then upload a binary
ELF image (copied as a raw file, since the ELF will be
loaded using RB's ELF loader) and leave the breakpoint
to continue booting.

From there the bootloader can load the ELF binary from
memory, exactly like a normal SD card boot.

Change-Id: I4eb971b4162ea422e38660455cfa0958cefaa18d
This commit is contained in:
Aidan MacDonald 2026-01-22 15:06:38 +00:00
parent 603c5bb2be
commit d5506dfa22
2 changed files with 90 additions and 3 deletions
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47
bootloader/echoplayer.c
View file

@ -41,6 +41,20 @@
#define LOAD_SIZE (2 * 1024 * 1024)
#define LOAD_BUFFER_ADDR (STM32_SDRAM1_BASE + SDRAM_SIZE - LOAD_SIZE)
/* Values at GDB_MAGICx */
#define GDB_MAGICVAL1 0x726f636b
#define GDB_MAGICVAL2 0x424f4f54
#define GDB_MAGICVAL3 0x6764626c
#define GDB_MAGICVAL4 0x6f616455
/* Addresses used by GDB boot protocol */
#define GDB_MAGIC1 (*(volatile uint32_t*)(STM32_SRAM4_BASE + 0x00))
#define GDB_MAGIC2 (*(volatile uint32_t*)(STM32_SRAM4_BASE + 0x04))
#define GDB_MAGIC3 (*(volatile uint32_t*)(STM32_SRAM4_BASE + 0x08))
#define GDB_MAGIC4 (*(volatile uint32_t*)(STM32_SRAM4_BASE + 0x0c))
#define GDB_ELFADDR (*(volatile uint32_t*)(STM32_SRAM4_BASE + 0x10))
#define GDB_ELFSIZE (*(volatile uint32_t*)(STM32_SRAM4_BASE + 0x14))
/* Events for the monitor callback to signal the main thread */
#define EV_POWER_PRESSED MAKE_SYS_EVENT(SYS_EVENT_CLS_PRIVATE, 0)
#define EV_POWER_RELEASED MAKE_SYS_EVENT(SYS_EVENT_CLS_PRIVATE, 1)
@ -320,6 +334,35 @@ static void launch(void)
launch_elf();
}
static bool handle_gdb_boot(void)
{
/* Look for magic values that signal the GDB boot protocol */
bool gdb_boot = (GDB_MAGIC1 == GDB_MAGICVAL1 &&
GDB_MAGIC2 == GDB_MAGICVAL2 &&
GDB_MAGIC3 == GDB_MAGICVAL3 &&
GDB_MAGIC4 == GDB_MAGICVAL4);
/* Clear them so they won't hang around on a system reset */
GDB_MAGIC1 = 0;
GDB_MAGIC2 = 0;
GDB_MAGIC3 = 0;
GDB_MAGIC4 = 0;
if (!gdb_boot)
return false;
/* "Call" GDB by entering breakpoint */
GDB_ELFADDR = 0;
GDB_ELFSIZE = 0;
asm volatile("bkpt");
/* Read location of the loaded binary */
elf_load_addr = (void *)GDB_ELFADDR;
elf_load_size = (size_t)GDB_ELFSIZE;
return true;
}
void main(void)
{
system_init();
@ -349,6 +392,10 @@ void main(void)
filesystem_init();
disk_mount_all();
/* GDB assisted boot takes precedence */
if (handle_gdb_boot())
launch_elf();
if (echoplayer_boot_reason == ECHOPLAYER_BOOT_REASON_SW_REBOOT ||
usb_detect() == USB_INSERTED)
{