[PATCH v2 4/4] serial: stm32: add FIFO threshold configuration

[Erwan Le Ray <erwan.leray@foss.st.com>]

From: Fabrice Gasnier <fabrice.gasnier@foss.st.com>

Add the support for two optional DT properties, to configure RX and TX
FIFO thresholds:
- rx-threshold
- tx-threshold
This replaces hard-coded 8 bytes threshold. Keep 8 as the default value if
not specified, for backward compatibility.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com>

Changes in v2:
Change added properties naming as proposed by Rob Herring.

diff --git a/drivers/tty/serial/stm32-usart.c b/drivers/tty/serial/stm32-usart.c
index 4d277804c63e..409cfd15a933 100644
--- a/drivers/tty/serial/stm32-usart.c
+++ b/drivers/tty/serial/stm32-usart.c
@@ -312,7 +312,7 @@ static void stm32_usart_tx_interrupt_enable(struct uart_port *port)
 	 * Enables TX FIFO threashold irq when FIFO is enabled,
 	 * or TX empty irq when FIFO is disabled
 	 */
-	if (stm32_port->fifoen)
+	if (stm32_port->fifoen && stm32_port->txftcfg >= 0)
 		stm32_usart_set_bits(port, ofs->cr3, USART_CR3_TXFTIE);
 	else
 		stm32_usart_set_bits(port, ofs->cr1, USART_CR1_TXEIE);
@@ -323,7 +323,7 @@ static void stm32_usart_tx_interrupt_disable(struct uart_port *port)
 	struct stm32_port *stm32_port = to_stm32_port(port);
 	const struct stm32_usart_offsets *ofs = &stm32_port->info->ofs;
 
-	if (stm32_port->fifoen)
+	if (stm32_port->fifoen && stm32_port->txftcfg >= 0)
 		stm32_usart_clr_bits(port, ofs->cr3, USART_CR3_TXFTIE);
 	else
 		stm32_usart_clr_bits(port, ofs->cr1, USART_CR1_TXEIE);
@@ -801,9 +801,10 @@ static void stm32_usart_set_termios(struct uart_port *port,
 	cr3 = readl_relaxed(port->membase + ofs->cr3);
 	cr3 &= USART_CR3_TXFTIE | USART_CR3_RXFTIE;
 	if (stm32_port->fifoen) {
-		cr3 &= ~(USART_CR3_TXFTCFG_MASK | USART_CR3_RXFTCFG_MASK);
-		cr3 |= USART_CR3_TXFTCFG_HALF << USART_CR3_TXFTCFG_SHIFT;
-		cr3 |= USART_CR3_RXFTCFG_HALF << USART_CR3_RXFTCFG_SHIFT;
+		if (stm32_port->txftcfg >= 0)
+			cr3 |= stm32_port->txftcfg << USART_CR3_TXFTCFG_SHIFT;
+		if (stm32_port->rxftcfg >= 0)
+			cr3 |= stm32_port->rxftcfg << USART_CR3_RXFTCFG_SHIFT;
 	}
 
 	if (cflag & CSTOPB)
@@ -833,7 +834,8 @@ static void stm32_usart_set_termios(struct uart_port *port,
 			, bits);
 
 	if (ofs->rtor != UNDEF_REG && (stm32_port->rx_ch ||
-				       stm32_port->fifoen)) {
+				       (stm32_port->fifoen &&
+					stm32_port->rxftcfg >= 0))) {
 		if (cflag & CSTOPB)
 			bits = bits + 3; /* 1 start bit + 2 stop bits */
 		else
@@ -1021,6 +1023,39 @@ static const struct uart_ops stm32_uart_ops = {
 	.verify_port	= stm32_usart_verify_port,
 };
 
+/*
+ * STM32H7 RX & TX FIFO threshold configuration (CR3 RXFTCFG / TXFTCFG)
+ * Note: 1 isn't a valid value in RXFTCFG / TXFTCFG. In this case,
+ * RXNEIE / TXEIE can be used instead of threshold irqs: RXFTIE / TXFTIE.
+ * So, RXFTCFG / TXFTCFG bitfields values are encoded as array index + 1.
+ */
+static const u32 stm32h7_usart_fifo_thresh_cfg[] = { 1, 2, 4, 8, 12, 14, 16 };
+
+static void stm32_usart_get_ftcfg(struct platform_device *pdev, const char *p,
+				  int *ftcfg)
+{
+	u32 bytes, i;
+
+	/* DT option to get RX & TX FIFO threshold (default to 8 bytes) */
+	if (of_property_read_u32(pdev->dev.of_node, p, &bytes))
+		bytes = 8;
+
+	for (i = 0; i < ARRAY_SIZE(stm32h7_usart_fifo_thresh_cfg); i++)
+		if (stm32h7_usart_fifo_thresh_cfg[i] >= bytes)
+			break;
+	if (i >= ARRAY_SIZE(stm32h7_usart_fifo_thresh_cfg))
+		i = ARRAY_SIZE(stm32h7_usart_fifo_thresh_cfg) - 1;
+
+	dev_dbg(&pdev->dev, "%s set to %d bytes\n", p,
+		stm32h7_usart_fifo_thresh_cfg[i]);
+
+	/* Provide FIFO threshold ftcfg (1 is invalid: threshold irq unused) */
+	if (i)
+		*ftcfg = i - 1;
+	else
+		*ftcfg = -EINVAL;
+}
+
 static void stm32_usart_deinit_port(struct stm32_port *stm32port)
 {
 	clk_disable_unprepare(stm32port->clk);
@@ -1057,6 +1092,12 @@ static int stm32_usart_init_port(struct stm32_port *stm32port,
 		of_property_read_bool(pdev->dev.of_node, "rx-tx-swap");
 
 	stm32port->fifoen = stm32port->info->cfg.has_fifo;
+	if (stm32port->fifoen) {
+		stm32_usart_get_ftcfg(pdev, "rx-threshold",
+				      &stm32port->rxftcfg);
+		stm32_usart_get_ftcfg(pdev, "tx-threshold",
+				      &stm32port->txftcfg);
+	}
 
 	res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
 	port->membase = devm_ioremap_resource(&pdev->dev, res);
diff --git a/drivers/tty/serial/stm32-usart.h b/drivers/tty/serial/stm32-usart.h
index 77d1ac082e89..07ac291328cd 100644
--- a/drivers/tty/serial/stm32-usart.h
+++ b/drivers/tty/serial/stm32-usart.h
@@ -216,12 +216,6 @@ struct stm32_usart_info stm32h7_info = {
 #define USART_CR3_TXFTCFG_MASK	GENMASK(31, 29)	/* H7 */
 #define USART_CR3_TXFTCFG_SHIFT	29		/* H7 */
 
-/* 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)
@@ -273,6 +267,8 @@ struct stm32_port {
 	bool hw_flow_control;
 	bool swap;		 /* swap RX & TX pins */
 	bool fifoen;
+	int rxftcfg;		/* RX FIFO threshold CFG      */
+	int txftcfg;		/* TX FIFO threshold CFG      */
 	bool wakeup_src;
 	int rdr_mask;		/* receive data register mask */
 	struct mctrl_gpios *gpios; /* modem control gpios */
-- 
2.17.1