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[uart] Wait for 16550 UART to become idle before modifying LCR

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Some implementations of 16550-compatible UARTs (e.g. the DesignWare
UART) are known to ignore writes to the line control register while
the transmitter is active.

Wait for the transmitter to become empty before attempting to write to
the line control register.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
This commit is contained in:
Michael Brown
2025-06-23 22:37:32 +01:00
parent 2ce1b185b2
commit 0ed1dea7f4
1 changed files with 3 additions and 0 deletions
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3
src/drivers/uart/ns16550.c
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@@ -131,6 +131,9 @@ static int ns16550_init ( struct uart *uart, unsigned int baud ) {
if ( ns16550_read ( ns16550, NS16550_SCR ) != 0xae )
return -ENODEV;
/* Wait for UART to become idle before modifying LCR */
ns16550_flush ( uart );
/* Configure divisor and line control register, if applicable */
ns16550_write ( ns16550, NS16550_LCR,
( NS16550_LCR_8N1 | NS16550_LCR_DLAB ) );