serial: zynqmp: Fetch baudrate from dtb and update

The baudrate configured in .config is taken by default by serial. If
change of baudrate is required then the .config needs to changed and
u-boot recompilation is required or the u-boot environment needs to be
updated.

To avoid this, support is added to fetch the baudrate directly from the
device tree file and update.
The serial, prints the log with the configured baudrate in the dtb.
The commit c4df0f6f315c ("arm: mvebu: Espressobin: Set default value for
$fdtfile env variable") is taken as reference for changing the default
environment variable.

The default environment stores the default baudrate value, When default
baudrate and dtb baudrate are not same glitches are seen on the serial.
So, the environment also needs to be updated with the dtb baudrate to
avoid the glitches on the serial.

Also add test to cover this new function.

Signed-off-by: Algapally Santosh Sagar <santoshsagar.algapally@amd.com>
Signed-off-by: Venkatesh Yadav Abbarapu <venkatesh.abbarapu@amd.com>
Link: https://lore.kernel.org/r/20230921112043.3144726-3-venkatesh.abbarapu@amd.com
Signed-off-by: Michal Simek <michal.simek@amd.com>

2 files changed, 58 insertions, 0 deletions

diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
index 8761a64..6628a88 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -24,6 +24,15 @@ config BAUDRATE
 	  in the SPL stage (most drivers) or for choosing a default baudrate
 	  in the absence of an environment setting (serial_mxc.c).
 
+config OF_SERIAL_BAUD
+	bool "Fetch serial baudrate from device tree"
+	depends on DM_SERIAL && SPL_ENV_SUPPORT
+	select DEFAULT_ENV_IS_RW
+	help
+	  Select this to enable fetching and setting of the baudrate
+	  configured in the DT. Replace the default baudrate with the DT
+	  baudrate and also set it to the environment.
+
 config DEFAULT_ENV_IS_RW
 	bool "Make default environment as writable"
 	help
diff --git a/drivers/serial/serial-uclass.c b/drivers/serial/serial-uclass.c
index df6a387..e4fa393 100644
--- a/drivers/serial/serial-uclass.c
+++ b/drivers/serial/serial-uclass.c
@@ -155,12 +155,61 @@ static void serial_find_console_or_panic(void)
 }
 #endif /* CONFIG_SERIAL_PRESENT */
 
+/**
+ * check_valid_baudrate() - Check whether baudrate is valid or not
+ *
+ * @baud: baud rate to check
+ * Return: 0 if OK, -ve on error
+ */
+static int check_valid_baudrate(int baud)
+{
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(baudrate_table); ++i) {
+		if (baud == baudrate_table[i])
+			return 0;
+	}
+
+	return -EINVAL;
+}
+
+int fetch_baud_from_dtb(void)
+{
+	int baud_value, ret;
+
+	baud_value = ofnode_read_baud();
+	ret = check_valid_baudrate(baud_value);
+	if (ret)
+		return ret;
+
+	return baud_value;
+}
+
 /* Called prior to relocation */
 int serial_init(void)
 {
 #if CONFIG_IS_ENABLED(SERIAL_PRESENT)
 	serial_find_console_or_panic();
 	gd->flags |= GD_FLG_SERIAL_READY;
+
+	if (IS_ENABLED(CONFIG_OF_SERIAL_BAUD)) {
+		int ret = 0;
+		char *ptr = (char*)&default_environment[0];
+
+		/*
+		 * Fetch the baudrate from the dtb and update the value in the
+		 * default environment.
+		 */
+		ret = fetch_baud_from_dtb();
+		if (ret != -EINVAL && ret != -EFAULT) {
+			gd->baudrate = ret;
+
+			while (*ptr != '\0' && *(ptr + 1) != '\0')
+				ptr++;
+			ptr += 2;
+			sprintf(ptr, "baudrate=%d", gd->baudrate);
+		}
+	}
 	serial_setbrg();
 #endif