Adding target drivers

Gareth McMullin edited this page Dec 1, 2016 · 7 revisions

This is a placeholder of cut-and-pasted text until I have a chance to write up some real documentation for the target API.

External target API

This is the public API, used by the GDB server. Nothing outside of this header should be accessed from outside of src/target/*

Internal target API

Structure definitions and convenience functions for use in target implementations. Specific target implementations fill in the function pointers in these structures for their device specific implementations

Raw JTAG Devices

Supported JTAG devices are defined here:

The .handler function is called when a device IDCODE matches .idcode with .idmask applied. It is the responsibility of the handler function to instantiate a new target with target_new and fill in the access methods.

These functions provide access to the JTAG IR and DR registers:

void jtag_dev_write_ir(jtag_dev_t *dev, uint32_t ir);
void jtag_dev_shift_dr(jtag_dev_t *dev, uint8_t *dout, const uint8_t *din, int ticks);

ARM implementations (ADIv5)

This table is a list of known Coresight CIDR and PIDR values:

The ROM table is read from the ADIv5 Mem-AP, and the appropriate probe function is called here:

There is currently support for:

The generic Cortex-M driver calls probe functions for each supported vendor device:

These probe functions should probe the target device, add any memories they support with target_add_ram and target_add_flash, and return true for any device they support, or return false otherwise.

Flash programming

The ADIv5 interface does not directly provide a way to write to flash. It's implemented differently for different target devices. This is done by filling in the function pointers in a struct target_flash structure and calling target_add_flash on the target in the device specific probe function.

struct target_flash {
    target_addr start;      /* Base address for this block of flash memory */
    size_t length;          /* Length of this block of flash memory */
    size_t blocksize;       /* Erase sector size for this block of flash memory */
    flash_erase_func erase; /* Function pointer to flash erase function.
                               Called with address and length aligned on .blocksize */
    flash_write_func write; /* Function pointer to flash write function.
                               Called with address and length aligned according to .align,
                               and padded with value in .erased. */
    flash_done_func done;   /* Called at the end of flash operations,
                               after all calls to .write */
    int align;              /* Alignment of buffer for .write */
    uint8_t erased;         /* Buffer pad value for .write */

There are a few basic patterns for how this is done in practice:

  • Direct - writes are passed directly to the target driver which programs the flash directly using MMIO. (eg. kinetis.c)
  • Buffered - the target layer will buffer write packets from GDB until and pass to the driver as writes of whole sectors. The driver will program the target flash directly using MMIO. This works well when whole sectors can be programmed with sequencial writes. (eg. stm32l0.c)
  • Stubbed - writes are passed directly to the target driver which writes a program stub and the data payload to the target. The stub is then executed to program the device. This works well when flash can't be programmed with sequential writes. (eg. stm32f1.c)
  • LPC - NXP devices include a built in ROM for programming flash. There is no published MMIO mechanism to program the flash on these devices. (eg. lpc11xx.c)

Buffered Flash model

For devices than can only efficiently program flash sectors of a fixed size there is some additional support in the target_flash structure. To make use of this mechanism, the .write and .done pointers must be assigned to target_flash_write_buffered and target_flash_done_buffered respectively. The .align field should be left set to zero when using the buffered flash model.

struct target_flash {
    /* For buffered flash */
    size_t buf_size;            /* Size of buffer for buffered operations */
    flash_write_func write_buf; /* Function pointer to buffered flash write function.
                                   Called with address aligned according to .buf_size,
                                   and padded with value in .erased.  The buffer
                                   will always be exactly .buf_size bytes */

Skeleton Driver

static int skeleton_flash_erase(struct target_flash *f,
                                target_addr addr, size_t len) {…}
static int skeleton_flash_write(struct target_flash *f,
                                target_addr dest, const void *src, size_t len) {…}

static void skeleton_add_flash(target *t)
    struct target_flash *f = calloc(1, sizeof(*f));
    f->start = SKELETON_FLASH_BASE;
    f->length = SKELETON_FLASH_SIZE;
    f->blocksize = SKELETON_BLOCKSIZE;
    f->erase = skeleton_flash_erase;
    f->write = skeleton_flash_write;
    target_add_flash(t, f);

bool skeleton_probe(target *t)
    if (target_mem_read32(t, SKELETON_DEVID_ADDR) == SKELETON_DEVID) {
        return true;
    } else {
        return false;
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