[PATCH v4 0/3] serial: 8250_dw: Fix ref clock usage

[PATCH v4 0/3] serial: 8250_dw: Fix ref clock usage

[Serge Semin]

Greg, Jiri, the merge window is upon us, please review/merge in/whatever
the rest of the patches.

It might be dangerous if an UART port reference clock rate is suddenly
changed. In particular the 8250 port drivers (and AFAICS most of the tty
drivers using common clock framework clocks) rely either on the
exclusive reference clock utilization or on the ref clock rate being
always constant. Needless to say that it turns out not true and if some
other service suddenly changes the clock rate behind an UART port driver
back it's no good. So the port might not only end up with an invalid
uartclk value saved, but may also experience a distorted output/input
data since such action will effectively update the programmed baud-clock.
We discovered such problem on Baikal-T1 SoC where two DW 8250 ports have
got a shared reference clock. Allwinner SoC is equipped with an UART,
which clock is derived from the CPU PLL clock source, so the CPU frequency
change might be propagated down up to the serial port reference clock.
This patchset provides a way to fix the problem to the 8250 serial port
controllers and mostly fixes it for the DW 8250-compatible UART. I say
mostly because due to not having a facility to pause/stop and resume/
restart on-going transfers we implemented the UART clock rate update
procedure executed post factum of the actual reference clock rate change.

In addition the patchset includes a few fixes we discovered when were
working the issue. First one concerns the maximum baud rate setting used
to determine a serial port baud based on the current UART port clock rate.
Another one simplifies the ref clock rate setting procedure a bit.

This patchset is rebased and tested on the mainline Linux kernel 5.7-rc4:
0e698dfa2822 ("Linux 5.7-rc4")
tag: v5.7-rc4

Changelog v3:
- Refactor the original patch to adjust the UART port divisor instead of
  requesting an exclusive ref clock utilization.

Changelog v4:
- Discard commit b426bf0fb085 ("serial: 8250: Fix max baud limit in generic
  8250 port") since Greg has already merged it into the tty-next branch.
- Use EXPORT_SYMBOL_GPL() for the serial8250_update_uartclk() method.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Cc: Maxim Kaurkin <Maxim.Kaurkin@baikalelectronics.ru>
Cc: Pavel Parkhomenko <Pavel.Parkhomenko@baikalelectronics.ru>
Cc: Alexey Kolotnikov <Alexey.Kolotnikov@baikalelectronics.ru>
Cc: Ramil Zaripov <Ramil.Zaripov@baikalelectronics.ru>
Cc: Ekaterina Skachko <Ekaterina.Skachko@baikalelectronics.ru>
Cc: Vadim Vlasov <V.Vlasov@baikalelectronics.ru>
Cc: Alexey Kolotnikov <Alexey.Kolotnikov@baikalelectronics.ru>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Will Deacon <will@kernel.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: linux-mips@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-serial@vger.kernel.org
Cc: linux-kernel@vger.kernel.org

Serge Semin (3):
  serial: 8250: Add 8250 port clock update method
  serial: 8250_dw: Simplify the ref clock rate setting procedure
  serial: 8250_dw: Fix common clocks usage race condition

 drivers/tty/serial/8250/8250_dw.c   | 125 +++++++++++++++++++++++++---
 drivers/tty/serial/8250/8250_port.c |  38 +++++++++
 include/linux/serial_8250.h         |   2 +
 3 files changed, 153 insertions(+), 12 deletions(-)

-- 
2.26.2

[PATCH v4 1/3] serial: 8250: Add 8250 port clock update method

[Serge Semin]

Some platforms can be designed in a way so the UART port reference clock
might be asynchronously changed at some point. In Baikal-T1 SoC this may
happen due to the reference clock being shared between two UART ports, on
the Allwinner SoC the reference clock is derived from the CPU clock, so
any CPU frequency change should get to be known/reflected by/in the UART
controller as well. But it's not enough to just update the
uart_port->uartclk field of the corresponding UART port, the 8250
controller reference clock divisor should be altered so to preserve
current baud rate setting. All of these things is done in a coherent
way by calling the serial8250_update_uartclk() method provided in this
patch. Though note that it isn't supposed to be called from within the
UART port callbacks because the locks using to the protect the UART port
data are already taken in there.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Will Deacon <will@kernel.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: linux-mips@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org

---

Changelog v4:
- Export serial8250_update_uartclk() symbol for GPL modules only.
---
 drivers/tty/serial/8250/8250_port.c | 38 +++++++++++++++++++++++++++++
 include/linux/serial_8250.h         |  2 ++
 2 files changed, 40 insertions(+)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 4d83c85a7389..5596868c8832 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2628,6 +2628,44 @@ static unsigned int serial8250_get_baud_rate(struct uart_port *port,
 				  (port->uartclk + tolerance) / 16);
 }
 
+/*
+ * Note in order to avoid the tty port mutex deadlock don't use the next method
+ * within the uart port callbacks. Primarily it's supposed to be utilized to
+ * handle a sudden reference clock rate change.
+ */
+void serial8250_update_uartclk(struct uart_port *port, unsigned int uartclk)
+{
+	struct uart_8250_port *up = up_to_u8250p(port);
+	unsigned int baud, quot, frac = 0;
+	struct ktermios *termios;
+	unsigned long flags;
+
+	mutex_lock(&port->state->port.mutex);
+
+	if (port->uartclk == uartclk)
+		goto out_lock;
+
+	port->uartclk = uartclk;
+	termios = &port->state->port.tty->termios;
+
+	baud = serial8250_get_baud_rate(port, termios, NULL);
+	quot = serial8250_get_divisor(port, baud, &frac);
+
+	spin_lock_irqsave(&port->lock, flags);
+
+	uart_update_timeout(port, termios->c_cflag, baud);
+
+	serial8250_set_divisor(port, baud, quot, frac);
+	serial_port_out(port, UART_LCR, up->lcr);
+	serial8250_out_MCR(up, UART_MCR_DTR | UART_MCR_RTS);
+
+	spin_unlock_irqrestore(&port->lock, flags);
+
+out_lock:
+	mutex_unlock(&port->state->port.mutex);
+}
+EXPORT_SYMBOL_GPL(serial8250_update_uartclk);
+
 void
 serial8250_do_set_termios(struct uart_port *port, struct ktermios *termios,
 			  struct ktermios *old)
diff --git a/include/linux/serial_8250.h b/include/linux/serial_8250.h
index 6545f8cfc8fa..2b70f736b091 100644
--- a/include/linux/serial_8250.h
+++ b/include/linux/serial_8250.h
@@ -155,6 +155,8 @@ extern int early_serial_setup(struct uart_port *port);
 
 extern int early_serial8250_setup(struct earlycon_device *device,
 					 const char *options);
+extern void serial8250_update_uartclk(struct uart_port *port,
+				      unsigned int uartclk);
 extern void serial8250_do_set_termios(struct uart_port *port,
 		struct ktermios *termios, struct ktermios *old);
 extern void serial8250_do_set_ldisc(struct uart_port *port,
-- 
2.26.2

[PATCH v4 2/3] serial: 8250_dw: Simplify the ref clock rate setting procedure

[Serge Semin]

Really instead of twice checking the clk_round_rate() return value
we could do it once, and if it isn't error the clock rate can be changed.
By doing so we decrease a number of ret-value tests and remove a weird
goto-based construction implemented in the dw8250_set_termios() method.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Will Deacon <will@kernel.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: linux-mips@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org

---

Changelog v3:
- This is a new patch.
---
 drivers/tty/serial/8250/8250_dw.c | 15 ++++-----------
 1 file changed, 4 insertions(+), 11 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
index aab3cccc6789..12866083731d 100644
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -282,20 +282,13 @@ static void dw8250_set_termios(struct uart_port *p, struct ktermios *termios,
 
 	clk_disable_unprepare(d->clk);
 	rate = clk_round_rate(d->clk, baud * 16);
-	if (rate < 0)
-		ret = rate;
-	else if (rate == 0)
-		ret = -ENOENT;
-	else
+	if (rate > 0) {
 		ret = clk_set_rate(d->clk, rate);
+		if (!ret)
+			p->uartclk = rate;
+	}
 	clk_prepare_enable(d->clk);
 
-	if (ret)
-		goto out;
-
-	p->uartclk = rate;
-
-out:
 	p->status &= ~UPSTAT_AUTOCTS;
 	if (termios->c_cflag & CRTSCTS)
 		p->status |= UPSTAT_AUTOCTS;
-- 
2.26.2

[PATCH v4 3/3] serial: 8250_dw: Fix common clocks usage race condition

[Serge Semin]

The race condition may happen if the UART reference clock is shared with
some other device (on Baikal-T1 SoC it's another DW UART port). In this
case if that device changes the clock rate while serial console is using
it the DW 8250 UART port might not only end up with an invalid uartclk
value saved, but may also experience a distorted output data since
baud-clock could have been changed. In order to fix this lets at least
try to adjust the 8250 port setting like UART clock rate in case if the
reference clock rate change is discovered. The driver will call the new
method to update 8250 UART port clock rate settings. It's done by means of
the clock event notifier registered at the port startup and unregistered
in the shutdown callback method.

Note 1. In order to avoid deadlocks we had to execute the UART port update
method in a dedicated deferred work. This is due to (in my opinion
redundant) the clock update implemented in the dw8250_set_termios()
method.
Note 2. Before the ref clock is manually changed by the custom
set_termios() function we swap the port uartclk value with new rate
adjusted to be suitable for the requested baud. It is necessary in
order to effectively disable a functionality of the ref clock events
handler for the current UART port, since uartclk update will be done
a bit further in the generic serial8250_do_set_termios() function.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Will Deacon <will@kernel.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: linux-mips@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org

---

Changelog v2:
- Move exclusive ref clock lock/unlock precudures to the 8250 port
  startup/shutdown methods.
- The changelog message has also been slightly modified due to the
  alteration.
- Remove Alexey' SoB tag.
- Cc someone from ARM who might be concerned regarding this change.
- Cc someone from Clocks Framework to get their comments on this patch.

Changelog v3:
- Refactor the original patch to adjust the UART port divisor instead of
  requesting an exclusive ref clock utilization.
---
 drivers/tty/serial/8250/8250_dw.c | 114 +++++++++++++++++++++++++++++-
 1 file changed, 111 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
index 12866083731d..cf4de510ab1b 100644
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -19,6 +19,8 @@
 #include <linux/of_irq.h>
 #include <linux/of_platform.h>
 #include <linux/platform_device.h>
+#include <linux/workqueue.h>
+#include <linux/notifier.h>
 #include <linux/slab.h>
 #include <linux/acpi.h>
 #include <linux/clk.h>
@@ -43,6 +45,8 @@ struct dw8250_data {
 	int			msr_mask_off;
 	struct clk		*clk;
 	struct clk		*pclk;
+	struct notifier_block	clk_notifier;
+	struct work_struct	clk_work;
 	struct reset_control	*rst;
 
 	unsigned int		skip_autocfg:1;
@@ -54,6 +58,16 @@ static inline struct dw8250_data *to_dw8250_data(struct dw8250_port_data *data)
 	return container_of(data, struct dw8250_data, data);
 }
 
+static inline struct dw8250_data *clk_to_dw8250_data(struct notifier_block *nb)
+{
+	return container_of(nb, struct dw8250_data, clk_notifier);
+}
+
+static inline struct dw8250_data *work_to_dw8250_data(struct work_struct *work)
+{
+	return container_of(work, struct dw8250_data, clk_work);
+}
+
 static inline int dw8250_modify_msr(struct uart_port *p, int offset, int value)
 {
 	struct dw8250_data *d = to_dw8250_data(p->private_data);
@@ -260,6 +274,45 @@ static int dw8250_handle_irq(struct uart_port *p)
 	return 0;
 }
 
+static void dw8250_clk_work_cb(struct work_struct *work)
+{
+	struct dw8250_data *d = work_to_dw8250_data(work);
+	struct uart_8250_port *up;
+	unsigned long rate;
+
+	rate = clk_get_rate(d->clk);
+	if (rate) {
+		up = serial8250_get_port(d->data.line);
+
+		serial8250_update_uartclk(&up->port, rate);
+	}
+}
+
+static int dw8250_clk_notifier_cb(struct notifier_block *nb,
+				  unsigned long event, void *data)
+{
+	struct dw8250_data *d = clk_to_dw8250_data(nb);
+
+	/*
+	 * We have no choice but to defer the uartclk update due to two
+	 * deadlocks. First one is caused by a recursive mutex lock which
+	 * happens when clk_set_rate() is called from dw8250_set_termios().
+	 * Second deadlock is more tricky and is caused by an inverted order of
+	 * the clk and tty-port mutexes lock. It happens if clock rate change
+	 * is requested asynchronously while set_termios() is executed between
+	 * tty-port mutex lock and clk_set_rate() function invocation and
+	 * vise-versa. Anyway if we didn't have the reference clock alteration
+	 * in the dw8250_set_termios() method we wouldn't have needed this
+	 * deferred event handling complication.
+	 */
+	if (event == POST_RATE_CHANGE) {
+		queue_work(system_unbound_wq, &d->clk_work);
+		return NOTIFY_OK;
+	}
+
+	return NOTIFY_DONE;
+}
+
 static void
 dw8250_do_pm(struct uart_port *port, unsigned int state, unsigned int old)
 {
@@ -283,9 +336,16 @@ static void dw8250_set_termios(struct uart_port *p, struct ktermios *termios,
 	clk_disable_unprepare(d->clk);
 	rate = clk_round_rate(d->clk, baud * 16);
 	if (rate > 0) {
-		ret = clk_set_rate(d->clk, rate);
-		if (!ret)
-			p->uartclk = rate;
+		/*
+		 * Premilinary set the uartclk to the new clock rate so the
+		 * clock update event handler caused by the clk_set_rate()
+		 * calling wouldn't actually update the UART divisor since
+		 * we about to do this anyway.
+		 */
+		swap(p->uartclk, rate);
+		ret = clk_set_rate(d->clk, p->uartclk);
+		if (ret)
+			swap(p->uartclk, rate);
 	}
 	clk_prepare_enable(d->clk);
 
@@ -312,6 +372,49 @@ static void dw8250_set_ldisc(struct uart_port *p, struct ktermios *termios)
 	serial8250_do_set_ldisc(p, termios);
 }
 
+static int dw8250_startup(struct uart_port *p)
+{
+	struct dw8250_data *d = to_dw8250_data(p->private_data);
+	int ret;
+
+	/*
+	 * Some platforms may provide a reference clock shared between several
+	 * devices. In this case before using the serial port first we have to
+	 * make sure that any clock state change is known to the UART port at
+	 * least post factum.
+	 */
+	if (d->clk) {
+		ret = clk_notifier_register(d->clk, &d->clk_notifier);
+		if (ret)
+			dev_warn(p->dev, "Failed to set the clock notifier\n");
+
+		/*
+		 * Get current reference clock rate to make sure the UART port
+		 * is equipped with an up-to-date value before it's started up.
+		 */
+		p->uartclk = clk_get_rate(d->clk);
+		if (!p->uartclk) {
+			dev_err(p->dev, "Clock rate not defined\n");
+			return -EINVAL;
+		}
+	}
+
+	return serial8250_do_startup(p);
+}
+
+static void dw8250_shutdown(struct uart_port *p)
+{
+	struct dw8250_data *d = to_dw8250_data(p->private_data);
+
+	serial8250_do_shutdown(p);
+
+	if (d->clk) {
+		clk_notifier_unregister(d->clk, &d->clk_notifier);
+
+		flush_work(&d->clk_work);
+	}
+}
+
 /*
  * dw8250_fallback_dma_filter will prevent the UART from getting just any free
  * channel on platforms that have DMA engines, but don't have any channels
@@ -407,6 +510,8 @@ static int dw8250_probe(struct platform_device *pdev)
 	p->serial_out	= dw8250_serial_out;
 	p->set_ldisc	= dw8250_set_ldisc;
 	p->set_termios	= dw8250_set_termios;
+	p->startup	= dw8250_startup;
+	p->shutdown	= dw8250_shutdown;
 
 	p->membase = devm_ioremap(dev, regs->start, resource_size(regs));
 	if (!p->membase)
@@ -468,6 +573,9 @@ static int dw8250_probe(struct platform_device *pdev)
 	if (IS_ERR(data->clk))
 		return PTR_ERR(data->clk);
 
+	INIT_WORK(&data->clk_work, dw8250_clk_work_cb);
+	data->clk_notifier.notifier_call = dw8250_clk_notifier_cb;
+
 	err = clk_prepare_enable(data->clk);
 	if (err)
 		dev_warn(dev, "could not enable optional baudclk: %d\n", err);
-- 
2.26.2

Re: [PATCH v4 3/3] serial: 8250_dw: Fix common clocks usage race condition

[Andy Shevchenko]

On Tue, May 26, 2020 at 07:03:16PM +0300, Serge Semin wrote:
> The race condition may happen if the UART reference clock is shared with
> some other device (on Baikal-T1 SoC it's another DW UART port). In this
> case if that device changes the clock rate while serial console is using
> it the DW 8250 UART port might not only end up with an invalid uartclk
> value saved, but may also experience a distorted output data since
> baud-clock could have been changed. In order to fix this lets at least
> try to adjust the 8250 port setting like UART clock rate in case if the
> reference clock rate change is discovered. The driver will call the new
> method to update 8250 UART port clock rate settings. It's done by means of
> the clock event notifier registered at the port startup and unregistered
> in the shutdown callback method.
> 
> Note 1. In order to avoid deadlocks we had to execute the UART port update
> method in a dedicated deferred work. This is due to (in my opinion
> redundant) the clock update implemented in the dw8250_set_termios()
> method.
> Note 2. Before the ref clock is manually changed by the custom
> set_termios() function we swap the port uartclk value with new rate
> adjusted to be suitable for the requested baud. It is necessary in
> order to effectively disable a functionality of the ref clock events
> handler for the current UART port, since uartclk update will be done
> a bit further in the generic serial8250_do_set_termios() function.

...

> +static void dw8250_clk_work_cb(struct work_struct *work)
> +{
> +	struct dw8250_data *d = work_to_dw8250_data(work);
> +	struct uart_8250_port *up;
> +	unsigned long rate;
> +
> +	rate = clk_get_rate(d->clk);

> +	if (rate) {

	if (rate <= 0)
		return;

?

> +		up = serial8250_get_port(d->data.line);
> +
> +		serial8250_update_uartclk(&up->port, rate);
> +	}
> +}

...

> +static int dw8250_startup(struct uart_port *p)
> +{
> +	struct dw8250_data *d = to_dw8250_data(p->private_data);
> +	int ret;
> +
> +	/*
> +	 * Some platforms may provide a reference clock shared between several
> +	 * devices. In this case before using the serial port first we have to
> +	 * make sure that any clock state change is known to the UART port at
> +	 * least post factum.
> +	 */

> +	if (d->clk) {

Do you need this?

> +		ret = clk_notifier_register(d->clk, &d->clk_notifier);

Okay, seems clk_notifier_register() and its counterpart should be fixed for
optional clocks.

> +		if (ret)
> +			dev_warn(p->dev, "Failed to set the clock notifier\n");

So, what does this warning mean on the platforms which does not need notifier
at all (i.o.w. all but baikal)?

> +		/*
> +		 * Get current reference clock rate to make sure the UART port
> +		 * is equipped with an up-to-date value before it's started up.
> +		 */

Why? We call ->set_termios() for it, no?

> +		p->uartclk = clk_get_rate(d->clk);
> +		if (!p->uartclk) {
> +			dev_err(p->dev, "Clock rate not defined\n");
> +			return -EINVAL;
> +		}
> +	}
> +
> +	return serial8250_do_startup(p);
> +}
> +
> +static void dw8250_shutdown(struct uart_port *p)
> +{
> +	struct dw8250_data *d = to_dw8250_data(p->private_data);
> +
> +	serial8250_do_shutdown(p);
> +

> +	if (d->clk) {

Ditto.

> +		clk_notifier_unregister(d->clk, &d->clk_notifier);
> +
> +		flush_work(&d->clk_work);
> +	}
> +}

-- 
With Best Regards,
Andy Shevchenko

Re: [PATCH v4 3/3] serial: 8250_dw: Fix common clocks usage race condition

[Serge Semin]

On Tue, May 26, 2020 at 07:57:01PM +0300, Andy Shevchenko wrote:
> On Tue, May 26, 2020 at 07:03:16PM +0300, Serge Semin wrote:
> > The race condition may happen if the UART reference clock is shared with
> > some other device (on Baikal-T1 SoC it's another DW UART port). In this
> > case if that device changes the clock rate while serial console is using
> > it the DW 8250 UART port might not only end up with an invalid uartclk
> > value saved, but may also experience a distorted output data since
> > baud-clock could have been changed. In order to fix this lets at least
> > try to adjust the 8250 port setting like UART clock rate in case if the
> > reference clock rate change is discovered. The driver will call the new
> > method to update 8250 UART port clock rate settings. It's done by means of
> > the clock event notifier registered at the port startup and unregistered
> > in the shutdown callback method.
> > 
> > Note 1. In order to avoid deadlocks we had to execute the UART port update
> > method in a dedicated deferred work. This is due to (in my opinion
> > redundant) the clock update implemented in the dw8250_set_termios()
> > method.
> > Note 2. Before the ref clock is manually changed by the custom
> > set_termios() function we swap the port uartclk value with new rate
> > adjusted to be suitable for the requested baud. It is necessary in
> > order to effectively disable a functionality of the ref clock events
> > handler for the current UART port, since uartclk update will be done
> > a bit further in the generic serial8250_do_set_termios() function.
> 
> ...
> 
> > +static void dw8250_clk_work_cb(struct work_struct *work)
> > +{
> > +	struct dw8250_data *d = work_to_dw8250_data(work);
> > +	struct uart_8250_port *up;
> > +	unsigned long rate;
> > +
> > +	rate = clk_get_rate(d->clk);
> 

> > +	if (rate) {
> 
> 	if (rate <= 0)
> 		return;
> 
> ?

Ok. Though there isn't point in a function consisting of a few lines. 

> 
> > +		up = serial8250_get_port(d->data.line);
> > +
> > +		serial8250_update_uartclk(&up->port, rate);
> > +	}
> > +}
> 
> ...
> 
> > +static int dw8250_startup(struct uart_port *p)
> > +{
> > +	struct dw8250_data *d = to_dw8250_data(p->private_data);
> > +	int ret;
> > +
> > +	/*
> > +	 * Some platforms may provide a reference clock shared between several
> > +	 * devices. In this case before using the serial port first we have to
> > +	 * make sure that any clock state change is known to the UART port at
> > +	 * least post factum.
> > +	 */
> 

> > +	if (d->clk) {
> 
> Do you need this?

Yes, I do. The same way as clk_get_rate() needs this.

> 
> > +		ret = clk_notifier_register(d->clk, &d->clk_notifier);
> 
> Okay, seems clk_notifier_register() and its counterpart should be fixed for
> optional clocks.

In order to use the clk_get_rate() function we need to make sure the clk isn't
optional otherwise -EINVAL will be returned, which is indistinguishable from
any another error. The same situation is for the clk_notifier_register() and
clk_notifier_unregister() counterpart.

> 
> > +		if (ret)
> > +			dev_warn(p->dev, "Failed to set the clock notifier\n");
> 
> So, what does this warning mean on the platforms which does not need notifier
> at all

It means "The clk-notifier subsystem is broken. Though if reference clock rate
doesn't change, it won't a problem." Due to the last statement we print a
warning, but not an error message.

> (i.o.w. all but baikal)?

No. As we discussed earlier in the previous pacthset versions there are another
platforms with shared reference clocks behind the DW APB UART, like: Allwinner SoCs,
RPi 3/4, etc.

> 
> > +		/*
> > +		 * Get current reference clock rate to make sure the UART port
> > +		 * is equipped with an up-to-date value before it's started up.
> > +		 */
> 
> Why? We call ->set_termios() for it, no?

This makes sense. Thanks. I'll remove this part.

-Sergey

> 
> > +		p->uartclk = clk_get_rate(d->clk);
> > +		if (!p->uartclk) {
> > +			dev_err(p->dev, "Clock rate not defined\n");
> > +			return -EINVAL;
> > +		}
> > +	}
> > +
> > +	return serial8250_do_startup(p);
> > +}
> > +
> > +static void dw8250_shutdown(struct uart_port *p)
> > +{
> > +	struct dw8250_data *d = to_dw8250_data(p->private_data);
> > +
> > +	serial8250_do_shutdown(p);
> > +
> 
> > +	if (d->clk) {
> 
> Ditto.
> 
> > +		clk_notifier_unregister(d->clk, &d->clk_notifier);
> > +
> > +		flush_work(&d->clk_work);
> > +	}
> > +}
> 
> -- 
> With Best Regards,
> Andy Shevchenko
> 
>