[PATCH v3] tty: serial: meson_uart: Add support for kernel debugger

[Julien Masson <jmasson@baylibre.com>]

The kgdb invokes the poll_put_char and poll_get_char when communicating
with the host. This patch implement the serial polling hooks for the
meson_uart to be used for KGDB debugging over serial line.

Signed-off-by: Julien Masson <jmasson@baylibre.com>
---

Changes since v2 [1]:
* Increase UART timeout to 10 ms
  -> For some reasons the previous value (1ms) is now too low with recent kernel.
     It made KGDB hang when printing long string for example.
     By setting this timeout to 10 ms, we avoid this kind of issue.

Changes since v1 [0]:
* Use readl_poll_timeout_atomic instead of looping with read + cpu_relax
  -> read every 5 usecs during 1 msec
* add some comments


* Test environment:
Board: "Le Potato"
https://libre.computer/products/boards/aml-s905x-cc/

Kernel Tree:
https://git.kernel.org/pub/scm/linux/kernel/git/khilman/linux-amlogic.git

Kernel command line arguments:
kgdboc=ttyAML0,115200 kgdbretry=4 nokaslr kgdbcon

Kernel modules:
CONFIG_DEBUG_INFO=y
CONFIG_DEBUG_KERNEL=y
CONFIG_FRAME_POINTER=y
CONFIG_KGDB=y
CONFIG_KGDB_SERIAL_CONSOLE=y

WARNING: for single step instruction I had to adapt/apply this patch:
https://lore.kernel.org/patchwork/patch/562423/


[0]: https://patchwork.kernel.org/patch/10792397/
[1]: https://patchwork.kernel.org/patch/10801583/

 drivers/tty/serial/meson_uart.c | 65 +++++++++++++++++++++++++++++++++
 1 file changed, 65 insertions(+)

diff --git a/drivers/tty/serial/meson_uart.c b/drivers/tty/serial/meson_uart.c
index fbc5bc022a39..b79c6d64bfb2 100644
--- a/drivers/tty/serial/meson_uart.c
+++ b/drivers/tty/serial/meson_uart.c
@@ -14,6 +14,7 @@
 #include <linux/delay.h>
 #include <linux/init.h>
 #include <linux/io.h>
+#include <linux/iopoll.h>
 #include <linux/module.h>
 #include <linux/kernel.h>
 #include <linux/of.h>
@@ -76,6 +77,8 @@
 #define AML_UART_PORT_OFFSET		6
 #define AML_UART_DEV_NAME		"ttyAML"
 
+#define AML_UART_POLL_USEC		5
+#define AML_UART_TIMEOUT_USEC		10000
 
 static struct uart_driver meson_uart_driver;
 
@@ -427,6 +430,64 @@ static void meson_uart_config_port(struct uart_port *port, int flags)
 	}
 }
 
+#ifdef CONFIG_CONSOLE_POLL
+/*
+ * Console polling routines for writing and reading from the uart while
+ * in an interrupt or debug context (i.e. kgdb).
+ */
+
+static int meson_uart_poll_get_char(struct uart_port *port)
+{
+	u32 c;
+	unsigned long flags;
+
+	spin_lock_irqsave(&port->lock, flags);
+
+	if (readl(port->membase + AML_UART_STATUS) & AML_UART_RX_EMPTY)
+		c = NO_POLL_CHAR;
+	else
+		c = readl(port->membase + AML_UART_RFIFO);
+
+	spin_unlock_irqrestore(&port->lock, flags);
+
+	return c;
+}
+
+static void meson_uart_poll_put_char(struct uart_port *port, unsigned char c)
+{
+	unsigned long flags;
+	u32 reg;
+	int ret;
+
+	spin_lock_irqsave(&port->lock, flags);
+
+	/* Wait until FIFO is empty or timeout */
+	ret = readl_poll_timeout_atomic(port->membase + AML_UART_STATUS, reg,
+					reg & AML_UART_TX_EMPTY,
+					AML_UART_POLL_USEC,
+					AML_UART_TIMEOUT_USEC);
+	if (ret == -ETIMEDOUT) {
+		dev_err(port->dev, "Timeout waiting for UART TX EMPTY\n");
+		goto out;
+	}
+
+	/* Write the character */
+	writel(c, port->membase + AML_UART_WFIFO);
+
+	/* Wait until FIFO is empty or timeout */
+	ret = readl_poll_timeout_atomic(port->membase + AML_UART_STATUS, reg,
+					reg & AML_UART_TX_EMPTY,
+					AML_UART_POLL_USEC,
+					AML_UART_TIMEOUT_USEC);
+	if (ret == -ETIMEDOUT)
+		dev_err(port->dev, "Timeout waiting for UART TX EMPTY\n");
+
+out:
+	spin_unlock_irqrestore(&port->lock, flags);
+}
+
+#endif /* CONFIG_CONSOLE_POLL */
+
 static const struct uart_ops meson_uart_ops = {
 	.set_mctrl      = meson_uart_set_mctrl,
 	.get_mctrl      = meson_uart_get_mctrl,
@@ -442,6 +503,10 @@ static const struct uart_ops meson_uart_ops = {
 	.request_port	= meson_uart_request_port,
 	.release_port	= meson_uart_release_port,
 	.verify_port	= meson_uart_verify_port,
+#ifdef CONFIG_CONSOLE_POLL
+	.poll_get_char	= meson_uart_poll_get_char,
+	.poll_put_char	= meson_uart_poll_put_char,
+#endif
 };
 
 #ifdef CONFIG_SERIAL_MESON_CONSOLE
-- 
2.21.1