nmaggioni commented on issue #16271:
URL: https://github.com/apache/nuttx/issues/16271#issuecomment-3263701404
> @linguini1 Have a bootloader that puts the RP2040 into BOOTSEL when it
detects something like 1200 baud on a UART line
In my RP2040 board I have implemented this over USB CDC-ACM instead of UART.
This comes at the cost of having a background demon that listens for
LINE_CODING events (let me improperly call them "baud rate changes"), though.
<details>
<summary>Example code, ran as part of bringup</summary>
Note: I have removed all error handling, comments, and mangled the control
structure of this code for maximum brevity. Consider it pseudocode at this
point :)
The `BOARDIOC_RESET_BOOTSEL` ioctl does what the name suggests and is backed
by a ROM lookup like in pico-sdk.
```c
static sem_t g_board_bootsel_on_baudrate_change_daemon_sem;
static void cdcacm_daemon_callback(enum cdcacm_event_e event) {
if (event == CDCACM_EVENT_LINECODING) {
sem_post(&g_board_bootsel_on_baudrate_change_daemon_sem);
}
}
static int cdcacm_daemon(int argc, char *argv[]) {
while (true) {
sem_wait(&g_board_bootsel_on_baudrate_change_daemon_sem);
int fd = open("/dev/ttyACM0", O_RDONLY);
struct cdc_linecoding_s linecoding;
int ret = ioctl(fd, CAIOC_GETLINECODING, &linecoding);
unsigned long baud = (linecoding.baud[3] << 24 | linecoding.baud[2]
<< 16 | linecoding.baud[1] << 8 | linecoding.baud[0]);
close(fd);
if (baud == 1200) {
boardctl(BOARDIOC_RESET_BOOTSEL, (uintptr_t)NULL);
}
}
}
static int board_cdcacm_daemon_setup(int argc, char *argv[]) {
uint8_t retry = 0;
uint8_t retry_delay_lut[] = { 1, 2, 3, 5, 10, 15, 30, 60 };
int fd;
while (true) {
fd = open("/dev/ttyACM0", O_RDONLY);
if (fd >= 0) break;
if (retry < ARRAY_SIZE(retry_delay_lut)) {
uint8_t delay = retry_delay_lut[retry++];
usleep(delay * USEC_PER_SEC);
} else {
return EXIT_FAILURE;
}
}
ioctl(fd, CAIOC_REGISTERCB, &cdcacm_daemon_callback);
close(fd);
sem_init(&g_board_bootsel_on_baudrate_change_daemon_sem, 0, 0);
sem_setprotocol(&g_board_bootsel_on_baudrate_change_daemon_sem,
SEM_PRIO_NONE);
task_create("cdcacm_daemon", sched_get_priority_min(SCHED_FIFO), 512,
cdcacm_daemon, NULL);
return EXIT_SUCCESS;
}
int board_app_finalinitialize(uintptr_t arg) {
int pid = task_create("board_cdcacm_daemon_setup",
sched_get_priority_min(SCHED_FIFO) + 1, 1024, board_cdcacm_daemon_setup, NULL);
return EXIT_SUCCESS;
}
```
</details>
```plaintext
nsh> ps
PID GROUP PRI POLICY TYPE NPX STATE EVENT STACK USED
FILLED COMMAND
0 0 0 FIFO Kthread - Ready 0001008 0000560
55.5% Idle_Task
1 1 100 RR Task - Running 0003032 0001664
54.8% nsh_main
3 3 1 RR Task - Waiting Semaphore 0000472 0000408
86.4%! cdcacm_daemon
```
This way I can run `stty -F /dev/serial/by-id/usb-<your_device_id> 1200` on
the host and have the board reboot to bootloader, and then it's just a matter
of a few shell commands to wait for the USB drive to appear and copy the new
UF2 file to it. I've wrapped all these actions in a couple of
[Just](https://just.systems/man/en/) recipes for ease of use, so I can get away
with things like `just build && just bootsel-flash` :)
@cederom What do you think of this approach? Would it be interesting to
upstream it or is it too niche? I typically use a debug probe while developing,
but this alternative way of flashing still comes in handy at times.
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