[PATCH 1/3] serial: 8250_dw: Keep the BUSY-safe divisor hook when DLF is present

[Vinicius Pedrosa]

dw8250_probe() installs dw8250_set_divisor(), which brackets the
DLAB/DLL/DLH sequence with dw8250_idle_enter()/dw8250_idle_exit() so that
a BUSY UART cannot drop the divisor LCR write. dw8250_setup_port() runs
afterwards and replaces that hook with the dwlib one whenever it detects
a DLF register. The dwlib hook programs the divisor through
serial8250_do_set_divisor() without entering idle.

When termios changes while the UART is transmitting, the LCR|DLAB write
is ignored and dw8250_check_lcr() retries it between
dw8250_idle_enter() and dw8250_idle_exit(). dw8250_idle_exit() restores
IER while DLAB is still set, so the value lands in DLH. The real IER
keeps the zero written by dw8250_idle_enter(), and the port gets no
further TX or RX interrupts while up->ier says they're enabled.

Program DLF from dw8250_set_divisor() inside the idle section and keep
that hook after dw8250_setup_port().

Observed on an Allwinner A733 (Radxa Cubie A7Z): the first interactive
tcsetattr() during console output left hardware IER at 0 and up->ier at
0x07. The shell never printed its prompt or read input. With this change
alone the same boot reached the prompt and echoed input, and hardware
IER matched up->ier afterwards.

Fixes: a7b9ce39fbe4 ("serial: 8250_dw: Ensure BUSY is deasserted")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Vinicius Pedrosa <vinicius.eduardo.pedrosa@xxxxxxxxx>
---
The A733 has no real DLF. Offset 0xc0 is an RS485 control register that
dw8250_setup_port() mistakes for one (patch 3), which is how this path
was reached. The new DLF write hasn't been tested on a UART with a real
DLF register.

 drivers/tty/serial/8250/8250_dw.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
index 0ab558230370..ba414306c98a 100644
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -189,6 +189,7 @@ static int dw8250_idle_enter(struct uart_port *p)
 static void dw8250_set_divisor(struct uart_port *p, unsigned int baud,
 			       unsigned int quot, unsigned int quot_frac)
 {
+	struct dw8250_data *d = to_dw8250_data(p->private_data);
 	struct uart_8250_port *up = up_to_u8250p(p);
 	int ret;
 
@@ -196,6 +197,9 @@ static void dw8250_set_divisor(struct uart_port *p, unsigned int baud,
 	if (ret < 0)
 		return;
 
+	if (d->data.dlf_size)
+		dw8250_writel_ext(p, DW_UART_DLF, quot_frac);
+
 	serial_port_out(p, UART_LCR, up->lcr | UART_LCR_DLAB);
 	if (!(serial_port_in(p, UART_LCR) & UART_LCR_DLAB))
 		goto idle_failed;
@@ -650,7 +654,6 @@ static int dw8250_probe(struct platform_device *pdev)
 
 	p->set_ldisc	= dw8250_set_ldisc;
 	p->set_termios	= dw8250_set_termios;
-	p->set_divisor	= dw8250_set_divisor;
 
 	data = devm_kzalloc(dev, sizeof(*data), GFP_KERNEL);
 	if (!data)
@@ -767,6 +770,13 @@ static int dw8250_probe(struct platform_device *pdev)
 	if (!data->skip_autocfg)
 		dw8250_setup_port(p);
 
+	/*
+	 * dw8250_setup_port() installs the library divisor hook when it finds
+	 * DLF. Keep the BUSY-safe hook instead: it also programs DLF, and it
+	 * must own DLAB so that a BUSY UART never drops the divisor LCR write.
+	 */
+	p->set_divisor = dw8250_set_divisor;
+
 	/* If we have a valid fifosize, try hooking up DMA */
 	if (p->fifosize) {
 		data->data.dma.rxconf.src_maxburst = p->fifosize / 4;
-- 
2.43.0