[PATCH 5.12 171/677] serial: stm32: fix tx_empty condition

[Greg Kroah-Hartman]

From: Erwan Le Ray <erwan.leray@xxxxxxxxxxx>

[ Upstream commit 3db1d52466dc11dca4e47ef12a6e6e97f846af62 ]

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: 48a6092fb41f ("serial: stm32-usart: Add STM32 USART Driver")
Signed-off-by: Erwan Le Ray <erwan.leray@xxxxxxxxxxx>
Link: https://lore.kernel.org/r/20210304162308.8984-13-erwan.leray@xxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 drivers/tty/serial/stm32-usart.c | 5 ++++-
 drivers/tty/serial/stm32-usart.h | 3 ---
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/serial/stm32-usart.c b/drivers/tty/serial/stm32-usart.c
index d205fce1950a..99dfa884cbef 100644
--- a/drivers/tty/serial/stm32-usart.c
+++ b/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)
diff --git a/drivers/tty/serial/stm32-usart.h b/drivers/tty/serial/stm32-usart.h
index cb4f327c46db..94b568aa46bb 100644
--- a/drivers/tty/serial/stm32-usart.h
+++ b/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)
 
-- 
2.30.2