tty: serial: meson_uart: Add support for kernel debugger

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>
Link: https://lore.kernel.org/r/867e1klo48.fsf@julienm-fedora-R90NQGV9.i-did-not-set--mail-host-address--so-tickle-me
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/meson_uart.c b/drivers/tty/serial/meson_uart.c
index 12e15358554cb3479626b52798655af8f13263f3..052c2e2fdd150cc7e7f3aca4244ee537e09549c2 100644 (file)
--- a/drivers/tty/serial/meson_uart.c
+++ b/drivers/tty/serial/meson_uart.c
@@ -10,6 +10,7 @@
 #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>
@@ -72,6 +73,8 @@
 #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;
 
@@ -423,6 +426,64 @@ 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,
@@ -438,6 +499,10 @@ 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