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Subject[PATCH 3.2 100/115] serial: sh-sci: Remove cpufreq notifier to fix crash/deadlock
3.2.80-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Geert Uytterhoeven <geert+renesas@glider.be>

commit ff1cab374ad98f4b9f408525ca9c08992b4ed784 upstream.

The BSP team noticed that there is spin/mutex lock issue on sh-sci when
CPUFREQ is used. The issue is that the notifier function may call
mutex_lock() while the spinlock is held, which can lead to a BUG().
This may happen if CPUFREQ is changed while another CPU calls
clk_get_rate().

Taking the spinlock was added to the notifier function in commit
e552de2413edad1a ("sh-sci: add platform device private data"), to
protect the list of serial ports against modification during traversal.
At that time the Common Clock Framework didn't exist yet, and
clk_get_rate() just returned clk->rate without taking a mutex.
Note that since commit d535a2305facf9b4 ("serial: sh-sci: Require a
device per port mapping."), there's no longer a list of serial ports to
traverse, and taking the spinlock became superfluous.

To fix the issue, just remove the cpufreq notifier:
1. The notifier doesn't work correctly: all it does is update the
stored clock rate; it does not update the divider in the hardware.
The divider will only be updated when calling sci_set_termios().
I believe this was broken back in 2004, when the old
drivers/char/sh-sci.c driver (where the notifier did update the
divider) was replaced by drivers/serial/sh-sci.c (where the
notifier just updated port->uartclk).
Cfr. full-history-linux commits 6f8deaef2e9675d9 ("[PATCH] sh: port
sh-sci driver to the new API") and 3f73fe878dc9210a ("[PATCH]
Remove old sh-sci driver").
2. On modern SoCs, the sh-sci parent clock rate is no longer related
to the CPU clock rate anyway, so using a cpufreq notifier is
futile.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
--- a/drivers/tty/serial/sh-sci.c
+++ b/drivers/tty/serial/sh-sci.c
@@ -40,7 +40,6 @@
#include <linux/console.h>
#include <linux/platform_device.h>
#include <linux/serial_sci.h>
-#include <linux/notifier.h>
#include <linux/pm_runtime.h>
#include <linux/cpufreq.h>
#include <linux/clk.h>
@@ -95,8 +94,6 @@ struct sci_port {
unsigned int rx_timeout;
#endif

- struct notifier_block freq_transition;
-
#ifdef CONFIG_SERIAL_SH_SCI_CONSOLE
unsigned short saved_smr;
unsigned short saved_fcr;
@@ -958,30 +955,6 @@ static irqreturn_t sci_mpxed_interrupt(i
return ret;
}

-/*
- * Here we define a transition notifier so that we can update all of our
- * ports' baud rate when the peripheral clock changes.
- */
-static int sci_notifier(struct notifier_block *self,
- unsigned long phase, void *p)
-{
- struct sci_port *sci_port;
- unsigned long flags;
-
- sci_port = container_of(self, struct sci_port, freq_transition);
-
- if ((phase == CPUFREQ_POSTCHANGE) ||
- (phase == CPUFREQ_RESUMECHANGE)) {
- struct uart_port *port = &sci_port->port;
-
- spin_lock_irqsave(&port->lock, flags);
- port->uartclk = clk_get_rate(sci_port->iclk);
- spin_unlock_irqrestore(&port->lock, flags);
- }
-
- return NOTIFY_OK;
-}
-
static struct sci_irq_desc {
const char *desc;
irq_handler_t handler;
@@ -2171,9 +2144,6 @@ static int sci_remove(struct platform_de
{
struct sci_port *port = platform_get_drvdata(dev);

- cpufreq_unregister_notifier(&port->freq_transition,
- CPUFREQ_TRANSITION_NOTIFIER);
-
uart_remove_one_port(&sci_uart_driver, &port->port);

clk_put(port->iclk);
@@ -2227,13 +2197,6 @@ static int __devinit sci_probe(struct pl
if (ret)
goto err_unreg;

- sp->freq_transition.notifier_call = sci_notifier;
-
- ret = cpufreq_register_notifier(&sp->freq_transition,
- CPUFREQ_TRANSITION_NOTIFIER);
- if (unlikely(ret < 0))
- goto err_unreg;
-
#ifdef CONFIG_SH_STANDARD_BIOS
sh_bios_gdb_detach();
#endif
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