serial: stm32: add support of RX FIFO threshold

Adds the support of RX FIFO threshold in order to improve the RX FIFO
management.
This is done by enabling fifo threshold interrupt, instead of relying
on rx empty/fifo not full irq. That basically generates one irq/char
currently. With this patch:
- RXCFG is set to half fifo size (e.g. 16/2 = 8 data for a 16 data depth
  FIFO)
- irq rate may be reduced by up to 1/RXCFG,  e.g. 1 over 8 with current
  RXCFG setting.
- Receiver timeout is used to gather chars when FIFO threshold isn't
  reached.
Signed-off-by: Erwan Le Ray <erwan.leray@st.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/stm32-usart.c

@@ -547,6 +547,9 @@ static void stm32_throttle(struct uart_port *port)
 
 	spin_lock_irqsave(&port->lock, flags);
 	stm32_clr_bits(port, ofs->cr1, stm32_port->cr1_irq);
+	if (stm32_port->cr3_irq)
+		stm32_clr_bits(port, ofs->cr3, stm32_port->cr3_irq);
+
 	spin_unlock_irqrestore(&port->lock, flags);
 }
 
@@ -559,6 +562,9 @@ static void stm32_unthrottle(struct uart_port *port)
 
 	spin_lock_irqsave(&port->lock, flags);
 	stm32_set_bits(port, ofs->cr1, stm32_port->cr1_irq);
+	if (stm32_port->cr3_irq)
+		stm32_set_bits(port, ofs->cr3, stm32_port->cr3_irq);
+
 	spin_unlock_irqrestore(&port->lock, flags);
 }
 
@@ -569,6 +575,9 @@ static void stm32_stop_rx(struct uart_port *port)
 	struct stm32_usart_offsets *ofs = &stm32_port->info->ofs;
 
 	stm32_clr_bits(port, ofs->cr1, stm32_port->cr1_irq);
+	if (stm32_port->cr3_irq)
+		stm32_clr_bits(port, ofs->cr3, stm32_port->cr3_irq);
+
 }
 
 /* Handle breaks - ignored by us */
@@ -597,8 +606,9 @@ static int stm32_startup(struct uart_port *port)
 
 	if (stm32_port->fifoen) {
 		val = readl_relaxed(port->membase + ofs->cr3);
-		val &= ~USART_CR3_TXFTCFG_MASK;
+		val &= ~(USART_CR3_TXFTCFG_MASK | USART_CR3_RXFTCFG_MASK);
 		val |= USART_CR3_TXFTCFG_HALF << USART_CR3_TXFTCFG_SHIFT;
+		val |= USART_CR3_RXFTCFG_HALF << USART_CR3_RXFTCFG_SHIFT;
 		writel_relaxed(val, port->membase + ofs->cr3);
 	}
 
@@ -690,7 +700,7 @@ static void stm32_set_termios(struct uart_port *port, struct ktermios *termios,
 		cr1 |= USART_CR1_FIFOEN;
 	cr2 = 0;
 	cr3 = readl_relaxed(port->membase + ofs->cr3);
-	cr3 &= USART_CR3_TXFTIE | USART_CR3_RXFTCFG | USART_CR3_RXFTIE
+	cr3 &= USART_CR3_TXFTIE | USART_CR3_RXFTCFG_MASK | USART_CR3_RXFTIE
 		| USART_CR3_TXFTCFG_MASK;
 
 	if (cflag & CSTOPB)
@@ -730,8 +740,14 @@ static void stm32_set_termios(struct uart_port *port, struct ktermios *termios,
 		stm32_port->cr1_irq = USART_CR1_RTOIE;
 		writel_relaxed(bits, port->membase + ofs->rtor);
 		cr2 |= USART_CR2_RTOEN;
+		/* Not using dma, enable fifo threshold irq */
+		if (!stm32_port->rx_ch)
+			stm32_port->cr3_irq =  USART_CR3_RXFTIE;
 	}
 
+	cr1 |= stm32_port->cr1_irq;
+	cr3 |= stm32_port->cr3_irq;
+
 	if (cflag & PARODD)
 		cr1 |= USART_CR1_PS;
 
@@ -973,6 +989,7 @@ static struct stm32_port *stm32_of_get_stm32_port(struct platform_device *pdev)
 							"st,hw-flow-ctrl");
 	stm32_ports[id].port.line = id;
 	stm32_ports[id].cr1_irq = USART_CR1_RXNEIE;
+	stm32_ports[id].cr3_irq = 0;
 	stm32_ports[id].last_res = RX_BUF_L;
 	return &stm32_ports[id];
 }

drivers/tty/serial/stm32-usart.h

@@ -210,7 +210,8 @@ struct stm32_usart_info stm32h7_info = {
 #define USART_CR3_WUFIE		BIT(22)		/* H7 */
 #define USART_CR3_TXFTIE	BIT(23)		/* H7 */
 #define USART_CR3_TCBGTIE	BIT(24)		/* H7 */
-#define USART_CR3_RXFTCFG	GENMASK(27, 25)	/* H7 */
+#define USART_CR3_RXFTCFG_MASK	GENMASK(27, 25)	/* H7 */
+#define USART_CR3_RXFTCFG_SHIFT	25		/* H7 */
 #define USART_CR3_RXFTIE	BIT(28)		/* H7 */
 #define USART_CR3_TXFTCFG_MASK	GENMASK(31, 29)	/* H7 */
 #define USART_CR3_TXFTCFG_SHIFT	29		/* H7 */
@@ -218,6 +219,9 @@ struct stm32_usart_info stm32h7_info = {
 /* TX FIFO threashold set to half of its depth */
 #define USART_CR3_TXFTCFG_HALF	0x2
 
+/* RX FIFO threashold set to half of its depth */
+#define USART_CR3_RXFTCFG_HALF	0x2
+
 /* USART_GTPR */
 #define USART_GTPR_PSC_MASK	GENMASK(7, 0)
 #define USART_GTPR_GT_MASK	GENMASK(15, 8)
@@ -263,6 +267,7 @@ struct stm32_port {
 	dma_addr_t tx_dma_buf;   /* dma tx buffer bus address */
 	unsigned char *tx_buf;   /* dma tx buffer cpu address */
 	u32 cr1_irq;		 /* USART_CR1_RXNEIE or RTOIE */
+	u32 cr3_irq;		 /* USART_CR3_RXFTIE */
 	int last_res;
 	bool tx_dma_busy;	 /* dma tx busy               */
 	bool hw_flow_control;