serial: stm32: fix tx_empty condition

In "tx_empty", we should poll TC bit in both DMA and PIO modes (instead of
TXE) to check transmission data register has been transmitted independently
of the FIFO mode. TC indicates that both transmit register and shift
register are empty. When shift register is empty, tx_empty should return
TIOCSER_TEMT instead of TC value.

Cleans the USART_CR_TC TCCF register define (transmission complete clear
flag) as it is duplicate of USART_ICR_TCCF.

Fixes: 48a6092fb4 ("serial: stm32-usart: Add STM32 USART Driver")
Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com>
Link: https://lore.kernel.org/r/20210304162308.8984-13-erwan.leray@foss.st.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/stm32-usart.c

@@ -515,7 +515,10 @@ static unsigned int stm32_usart_tx_empty(struct uart_port *port)
 	struct stm32_port *stm32_port = to_stm32_port(port);
 	const struct stm32_usart_offsets *ofs = &stm32_port->info->ofs;
 
-	return readl_relaxed(port->membase + ofs->isr) & USART_SR_TXE;
+	if (readl_relaxed(port->membase + ofs->isr) & USART_SR_TC)
+		return TIOCSER_TEMT;
+
+	return 0;
 }
 
 static void stm32_usart_set_mctrl(struct uart_port *port, unsigned int mctrl)

drivers/tty/serial/stm32-usart.h

@@ -127,9 +127,6 @@ struct stm32_usart_info stm32h7_info = {
 /* Dummy bits */
 #define USART_SR_DUMMY_RX	BIT(16)
 
-/* USART_ICR (F7) */
-#define USART_CR_TC		BIT(6)
-
 /* USART_DR */
 #define USART_DR_MASK		GENMASK(8, 0)