[U-Boot] [PATCH v2] watchdog: Fix Watchdog Reset while in U-Boot Prompt

[Andreas J. Reichel]

This patch fixes unwanted watchdog resets while the user enters
a command at the U-Boot prompt.

As found on the CM-FX6 board from Compulab, when having enabled the
watchdog, a missing WATCHDOG_RESET call in _serial_tstc() in
(/drivers/serial/serial-uclass.c) causes this and alike boards to
reset when the watchdog's timeout has elapsed while waiting at the
U-Boot prompt.

Despite the user could press several keys within the watchdog
timeout limit, the while loop in cli_readline.c, line 261, does only
call WATCHDOG_RESET if first == 1, which gets set to 0 in the 1st
loop iteration. This leads to a watchdog timeout no matter if the
user presses keys or not.

The problem is solved with a call to WATCHDOG_RESET in _serial_tstc,
defined in drivers/serial/serial-uclass.c.

Since the macro WATCHDOG_RESET expands to {} if watchdog support
isn't configured, there's no need to surround it by #ifdef in this
case.

 * Symptom:
   U-Boot resets after watchdog times out when in commandline prompt
   and watchdog is enabled.

 * Reasoning:
   When U-Boot shows the commandline prompt, the following function
   call stack is executed while waiting for a keypress:

   common/main.c:
                    main_loop          => common/cli.c: cli_loop() =>
   common/cli_hush.c:
                    parse_file_outer   => parse_stream_outer       =>
                    parse_stream       => b_getch(i)               =>
                    i->get(i)          => file_get                 =>
                    get_user_input     => cmdedit_read_input       =>
                    uboot_cli_readline =>
   common/cli_readline.c:
                    cli_readline       => cli_readline_into_buffer =>
                    cread_line         => getcmd_getch (== getc)   =>
   commonn/console.c:
                    fgetc              => console_tstc

   - in console_tstc line 181:
   If dev->tstc(dev) returns 0, the global tstcdev variable doesn't
   get set. This is the case if no character is in the serial buffer.

   - in fgetc(int file), line 297:
   Program flow keeps looping because tstcdev does not get set.
   Therefore WATCHDOG_RESET is not called, as mx_serial_tstc from
   drivers/serial/serial_mxc.c does not call it.

   - Initialization calls drv_system_init in stdio.c, which sets
   dev.tstc = stdio_serial_tstc. Thus, dev->tstc(dev) calls serial_tstc()
   which in turn calls _serial_tstc().

   Hence, _serial_tstc() needs to call WATCHDOG_RESET() to periodically
   reset the watchdog while cli_readline waits for user input.

Signed-off-by: Christian Storm <christian.storm at tngtech.com>
Signed-off-by: Jan Kiszka <jan.kiszka at siemens.com>
Signed-off-by: Andreas J. Reichel <Andreas.Reichel at tngtech.com>
---

Changes in v2:
 - Move WATCHDOG_RESET() call from common/console.c to
   drivers/serial/serial-uclass.c.

 drivers/serial/serial-uclass.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/serial/serial-uclass.c b/drivers/serial/serial-uclass.c
index 0ce5c44..72cf808 100644
--- a/drivers/serial/serial-uclass.c
+++ b/drivers/serial/serial-uclass.c
@@ -155,6 +155,7 @@ static int _serial_tstc(struct udevice *dev)
 {
 	struct dm_serial_ops *ops = serial_get_ops(dev);
 
+	WATCHDOG_RESET();
 	if (ops->pending)
 		return ops->pending(dev, true);
 
-- 
2.8.2