From: Michael Brown Date: Mon, 23 Jun 2025 21:37:32 +0000 (+0100) Subject: [uart] Wait for 16550 UART to become idle before modifying LCR X-Git-Url: http://git.ipfire.org/gitweb/gitweb.cgi?a=commitdiff_plain;h=0ed1dea7f4f5c5b2b7cd0130eb49c2c5c0b0ea42;p=thirdparty%2Fipxe.git [uart] Wait for 16550 UART to become idle before modifying LCR 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 --- diff --git a/src/drivers/uart/ns16550.c b/src/drivers/uart/ns16550.c index cf8c744c6..5455c68fb 100644 --- a/src/drivers/uart/ns16550.c +++ b/src/drivers/uart/ns16550.c @@ -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 ) );