cc2538: uart: Fix crippled output occurring upon lpm_enter()

lpm_enter() must not enter PM1+ if the UART TX FIFO is not empty. Otherwise, the
UART clock gets disabled, and its TX is broken.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>

cpu/cc2538/dev/uart.c

@@ -41,8 +41,10 @@
 #include "dev/ioc.h"
 #include "dev/gpio.h"
 #include "dev/uart.h"
+#include "lpm.h"
 #include "reg.h"
 
+#include <stdbool.h>
 #include <stdint.h>
 #include <string.h>
 
@@ -96,9 +98,19 @@ reset(void)
   REG(UART_BASE | UART_CTL) |= UART_CTL_UARTEN;
 }
 /*---------------------------------------------------------------------------*/
+static bool
+permit_pm1(void)
+{
+  /* Note: UART_FR.TXFE reads 0 if the UART clock is gated. */
+  return (REG(SYS_CTRL_RCGCUART) & SYS_CTRL_RCGCUART_UART) == 0 ||
+         (REG(UART_BASE | UART_FR) & UART_FR_TXFE) != 0;
+}
+/*---------------------------------------------------------------------------*/
 void
 uart_init(void)
 {
+  lpm_register_peripheral(permit_pm1);
+
   /* Enable clock for the UART while Running, in Sleep and Deep Sleep */
   REG(SYS_CTRL_RCGCUART) |= SYS_CTRL_RCGCUART_UART;
   REG(SYS_CTRL_SCGCUART) |= SYS_CTRL_SCGCUART_UART;

cpu/cc2538/lpm.c

@@ -102,7 +102,7 @@ static uint8_t max_pm;
 #ifdef LPM_CONF_PERIPH_PERMIT_PM1_FUNCS_MAX
 #define LPM_PERIPH_PERMIT_PM1_FUNCS_MAX LPM_CONF_PERIPH_PERMIT_PM1_FUNCS_MAX
 #else
-#define LPM_PERIPH_PERMIT_PM1_FUNCS_MAX 1
+#define LPM_PERIPH_PERMIT_PM1_FUNCS_MAX 2
 #endif
 
 lpm_periph_permit_pm1_func_t

cpu/cc2538/lpm.h

@@ -182,7 +182,7 @@ typedef bool (*lpm_periph_permit_pm1_func_t)(void);
  * \param permit_pm1_func Pointer to the function
  *
  * Some peripherals are sensitive to PM changes. For instance, we don't want to
- * drop to PM1+ if the USB PLL is active.
+ * drop to PM1+ if the USB PLL is active or if the UART TX FIFO is not clear.
  *
  * When changing power modes, the LPM driver will call all FPs registered with
  * this function. The peripheral's function will return true or false to permit