[U-Boot] serial: ns16550: fix debug uart putc called before init

[Tom Rini]

On Wed, Jan 09, 2019 at 08:35:31PM +0100, Simon Goldschmidt wrote:

> 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 at gmail.com>
> Reviewed-by: Simon Glass <sjg at chromium.org>

Applied to u-boot/master, thanks!

-- 
Tom
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