src/cyw43_ctrl: Lock the bus across BT HCI read/write.

cyw43_bluetooth_hci_read() and cyw43_bluetooth_hci_write() call
cyw43_btbus_read()/cyw43_btbus_write() without holding the cyw43 bus
lock. The BT shared bus runs over the same gSPI/SDIO backplane as WiFi,
so a caller that does not already serialise against WiFi will interleave
WiFi and BT transfers on the bus.

The btstack HCI transport in pico-sdk already wraps these calls in
CYW43_THREAD_ENTER/EXIT, so it is unaffected. But any other consumer
that calls the BT HCI API directly (e.g. a non-btstack or multi-threaded
port) is left unprotected. Take CYW43_THREAD_ENTER across the whole
transfer so the functions are self-serialising. The lock is recursive,
so the existing wrapping in the btstack transport and the nested
cyw43_ensure_bt_up() continue to work unchanged.

Signed-off-by: Jonathan Beri <jmberi@gmail.com>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: beriberikix

src/cyw43_ctrl.c

@@ -782,30 +782,39 @@ int cyw43_bluetooth_hci_init(void) {
 // Read data
 int cyw43_bluetooth_hci_read(uint8_t *buf, uint32_t max_size, uint32_t *len) {
     cyw43_t *self = &cyw43_state;
+    // Hold the bus lock across the whole transfer: the BT shared bus runs over
+    // the same gSPI/SDIO backplane as WiFi, so a caller that does not already
+    // serialise against WiFi (i.e. anything other than the btstack transport)
+    // would otherwise interleave WiFi and BT transfers on the bus.
+    CYW43_THREAD_ENTER;
     int ret = cyw43_ensure_bt_up(self);
     if (ret) {
+        CYW43_THREAD_EXIT;
         return ret;
     }
     ret = cyw43_btbus_read(buf, max_size, len);
     if (ret) {
         CYW43_PRINTF("cyw43_bluetooth_hci_read: failed to read from shared bus\n");
-        return ret;
     }
-    return 0;
+    CYW43_THREAD_EXIT;
+    return ret;
 }
 
 // Write data, buffer needs to include space for a 4 byte header and include this in len
 int cyw43_bluetooth_hci_write(uint8_t *buf, size_t len) {
     cyw43_t *self = &cyw43_state;
+    // See cyw43_bluetooth_hci_read: hold the bus lock across the whole transfer.
+    CYW43_THREAD_ENTER;
     int ret = cyw43_ensure_bt_up(self);
     if (ret) {
+        CYW43_THREAD_EXIT;
         return ret;
     }
     ret = cyw43_btbus_write(buf, len);
     if (ret) {
         CYW43_PRINTF("cyw43_bluetooth_hci_write: failed to write to shared bus\n");
-        return ret;
     }
-    return 0;
+    CYW43_THREAD_EXIT;
+    return ret;
 }
 #endif