[PATCH v2 1/4] serial: 8250_dw: Keep the BUSY-safe divisor hook when DLF is present

[Vinicius Pedrosa <vinicius.eduardo.pedrosa@gmail.com>]

dw8250_probe() installs dw8250_set_divisor(), which brackets the
DLAB/DLL/DLH sequence with dw8250_idle_enter() and dw8250_idle_exit()
so a BUSY UART cannot drop the divisor LCR write.

dw8250_setup_port() runs afterwards and replaces that hook with the
dwlib divisor hook whenever it detects DLF. The dwlib hook programs the
divisor through serial8250_do_set_divisor() without entering idle,
leaving the LCR|DLAB write exposed to BUSY.

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

Do not replace a divisor hook the driver has already installed. Program
DLF from the BUSY-safe hook, inside the idle section, through a dwlib
helper that checks dlf_size.

Observed on an Allwinner A733 (Radxa Cubie A7Z), where
dw8250_setup_port() mistakes the RS485 control register for DLF (see the
next patch): 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@vger.kernel.org
Signed-off-by: Vinicius Pedrosa <vinicius.eduardo.pedrosa@gmail.com>
---
 drivers/tty/serial/8250/8250_dw.c    |  2 ++
 drivers/tty/serial/8250/8250_dwlib.c | 16 ++++++++++++++--
 drivers/tty/serial/8250/8250_dwlib.h |  1 +
 3 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
index eca02e8bd6d1..4c86aaf9c121 100644
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -197,6 +197,8 @@ static void dw8250_set_divisor(struct uart_port *p, unsigned int baud,
 	if (ret < 0)
 		return;
 
+	__dw8250_set_divisor(p, 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;
diff --git a/drivers/tty/serial/8250/8250_dwlib.c b/drivers/tty/serial/8250/8250_dwlib.c
index 9bb02a4ab11f..325793f20256 100644
--- a/drivers/tty/serial/8250/8250_dwlib.c
+++ b/drivers/tty/serial/8250/8250_dwlib.c
@@ -36,10 +36,20 @@ static unsigned int dw8250_get_divisor(struct uart_port *p, unsigned int baud,
 	return quot;
 }
 
+/* Program the fractional part of the divisor, if the UART has one */
+void __dw8250_set_divisor(struct uart_port *p, unsigned int quot_frac)
+{
+	struct dw8250_port_data *d = p->private_data;
+
+	if (d->dlf_size)
+		dw8250_writel_ext(p, DW_UART_DLF, quot_frac);
+}
+EXPORT_SYMBOL_GPL(__dw8250_set_divisor);
+
 static void dw8250_set_divisor(struct uart_port *p, unsigned int baud,
 			       unsigned int quot, unsigned int quot_frac)
 {
-	dw8250_writel_ext(p, DW_UART_DLF, quot_frac);
+	__dw8250_set_divisor(p, quot_frac);
 	serial8250_do_set_divisor(p, baud, quot);
 }
 
@@ -218,7 +228,9 @@ void dw8250_setup_port(struct uart_port *p)
 	if (reg) {
 		pd->dlf_size = fls(reg);
 		p->get_divisor = dw8250_get_divisor;
-		p->set_divisor = dw8250_set_divisor;
+		/* Keep a hook the driver has already installed */
+		if (!p->set_divisor)
+			p->set_divisor = dw8250_set_divisor;
 	}
 
 	reg = dw8250_readl_ext(p, DW_UART_UCV);
diff --git a/drivers/tty/serial/8250/8250_dwlib.h b/drivers/tty/serial/8250/8250_dwlib.h
index 39fcb1b323ad..4bb0ccb3e8ef 100644
--- a/drivers/tty/serial/8250/8250_dwlib.h
+++ b/drivers/tty/serial/8250/8250_dwlib.h
@@ -99,6 +99,7 @@ struct dw8250_port_data {
 
 void dw8250_do_set_termios(struct uart_port *p, struct ktermios *termios, const struct ktermios *old);
 void dw8250_setup_port(struct uart_port *p);
+void __dw8250_set_divisor(struct uart_port *p, unsigned int quot_frac);
 
 static inline u32 dw8250_readl_ext(struct uart_port *p, int offset)
 {