Define target baud rate for UART negotiation

Added target baud rate for UART negotiation and updated comments.

Signed-off-by: Niel Nielsen <nieldk@gmail.com>

armsrc/bwm_uart_at32.h

@@ -14,11 +14,27 @@
 
 #include "common.h"
 
-#define BWM_UART_BAUD   460800
+// Boot/default baud. Must match UART_BAUD_RATE_DEFAULT in the ESP firmware
+// (Proxmark5_BWM_esp32). The link comes up here, then the ARM negotiates up to
+// BWM_UART_BAUD_TARGET at runtime via app_com cmd 1011 - the ESP is designed to
+// be told, not reflashed (the SET command is explicitly non-persistent).
+#define BWM_UART_BAUD          460800
+
+// Target baud to negotiate after link-up. 0 disables negotiation (stay at
+// BWM_UART_BAUD). Bounded by the ESP's CONFIG_SOC_UART_BITRATE_MAX and PA0/PA1
+// signal integrity; validate on hardware before raising further.
+#ifndef BWM_UART_BAUD_TARGET
+#define BWM_UART_BAUD_TARGET   921600
+#endif
 
 void bwm_uart_init(void);
 
-void UART4_IRQHandler(void);
+// Re-init UART4 + RX DMA at a new baud (used by the runtime baud negotiation).
+// Discards any bytes already in the RX ring.
+void bwm_uart_set_baud(uint32_t baud);
+
+// Baud the link is currently running at (updated by bwm_uart_set_baud).
+uint32_t bwm_uart_get_baud(void);
 
 int bwm_uart_write(const uint8_t *data, size_t len);