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serial: ns16550: fix debug uart putc called before init

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If _debug_uart_putc() is called before _debug_uart_init(), the
ns16550 debug uart driver hangs in a tight loop waiting for the
tx FIFO to get empty.

As this can happen via a printf sneaking in before the port calls
debug_uart_init(), introduce a config option to ignore characters
before the debug uart is initialized.

This is done by reading the baudrate divisor and aborting if is zero.

The Kconfig option is required as reading the baudrate divisor does
not seem to work for all ns16500 compatibles (which is why the last
attempt on this has been reverted in 1a67969a99).

Tested on socfpga_cyclone5_socrates.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
This commit is contained in:
Simon Goldschmidt 2019-01-09 20:35:31 +01:00 committed by Tom Rini
parent 7828e3cf70
commit c4448bdc4f
2 changed files with 30 additions and 2 deletions
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12
drivers/serial/Kconfig
View File

@ -447,6 +447,18 @@ config DEBUG_UART_SKIP_INIT
Select this if the UART you want to use for debug output is already
initialized by the time U-Boot starts its execution.
config DEBUG_UART_NS16550_CHECK_ENABLED
bool "Check if UART is enabled on output"
depends on DEBUG_UART
depends on DEBUG_UART_NS16550
help
Select this if puts()/putc() might be called before the debug UART
has been initialized. If this is disabled, putc() might sit in a
tight loop if it is called before debug_uart_init() has been called.
Note that this does not work for every ns16550-compatible UART and
so has to be enabled carefully or you might notice lost characters.
config ALTERA_JTAG_UART
bool "Altera JTAG UART support"
depends on DM_SERIAL

20
drivers/serial/ns16550.c
View File

@ -272,12 +272,28 @@ static inline void _debug_uart_init(void)
serial_dout(&com_port->lcr, UART_LCRVAL);
}
static inline int NS16550_read_baud_divisor(struct NS16550 *com_port)
{
int ret;
serial_dout(&com_port->lcr, UART_LCR_BKSE | UART_LCRVAL);
ret = serial_din(&com_port->dll) & 0xff;
ret |= (serial_din(&com_port->dlm) & 0xff) << 8;
serial_dout(&com_port->lcr, UART_LCRVAL);
return ret;
}
static inline void _debug_uart_putc(int ch)
{
struct NS16550 *com_port = (struct NS16550 *)CONFIG_DEBUG_UART_BASE;
while (!(serial_din(&com_port->lsr) & UART_LSR_THRE))
;
while (!(serial_din(&com_port->lsr) & UART_LSR_THRE)) {
#ifdef CONFIG_DEBUG_UART_NS16550_CHECK_ENABLED
if (!NS16550_read_baud_divisor(com_port))
return;
#endif
}
serial_dout(&com_port->thr, ch);
}