serial: lpuart: test the receive flag, not the FIFO counter

On the 8 bit register variant, the only user of which is fsl,vf610-lpuart,
_lpuart_serial_init() disables the FIFO:

	/* Disable FIFO and flush buffer */
	__raw_writeb(0x0, &base->upfifo);

while _lpuart_serial_tstc() asks the receive FIFO counter whether a
character arrived:

	if (__raw_readb(&base->urcfifo) == 0)
		return 0;

With the FIFO disabled the dataword lands in the data register and raises
S1[RDRF], but RCFIFO stays zero, so tstc() never reports a character. The
console is then output only: autoboot cannot be interrupted, ctrlc() never
fires, and anything polling for a key waits forever.

_lpuart_serial_getc() in the same driver already tests S1, and the 32 bit
variant enables its FIFO in _lpuart32_serial_init() before reading the
RXCOUNT field of WATER, so only this path is inconsistent. Test S1 the way
getc() does.

Tested on a Colibri VF50: U-Boot printed its whole log over UART0 but
accepted no input at all, neither a key during the boot delay nor Ctrl-C
during a sleep, while Linux received on the same pads with the same pin
mux. With the fix the boot delay can be interrupted, including with
bootdelay=0, where the key is already buffered when abortboot_single_key()
checks.

Signed-off-by: Mehmet Fide <mehmet.fide@screeningeagle.com>

1 files changed, 5 insertions, 4 deletions

diff --git a/drivers/serial/serial_lpuart.c b/drivers/serial/serial_lpuart.c
index 917445e1ee2..dccd0974150 100644
--- a/drivers/serial/serial_lpuart.c
+++ b/drivers/serial/serial_lpuart.c
@@ -194,10 +194,11 @@ static int _lpuart_serial_tstc(struct lpuart_serial_plat *plat)
 {
 	struct lpuart_fsl *base = plat->reg;
 
-	if (__raw_readb(&base->urcfifo) == 0)
-		return 0;
-
-	return 1;
+	/*
+	 * The receive FIFO counter stays at zero because _lpuart_serial_init()
+	 * disables the FIFO, so ask the status register, the way getc() does.
+	 */
+	return __raw_readb(&base->us1) & (US1_RDRF | US1_OR) ? 1 : 0;
 }
 
 /*