pinctrl: mcp23s08: Fix sleeping in atomic context due to regmap locking

If a device uses MCP23xxx IO expander to receive IRQs, the following
bug can happen:

  BUG: sleeping function called from invalid context
    at kernel/locking/mutex.c:283
  in_atomic(): 1, irqs_disabled(): 1, non_block: 0, ...
  preempt_count: 1, expected: 0
  ...
  Call Trace:
  ...
  __might_resched+0x104/0x10e
  __might_sleep+0x3e/0x62
  mutex_lock+0x20/0x4c
  regmap_lock_mutex+0x10/0x18
  regmap_update_bits_base+0x2c/0x66
  mcp23s08_irq_set_type+0x1ae/0x1d6
  __irq_set_trigger+0x56/0x172
  __setup_irq+0x1e6/0x646
  request_threaded_irq+0xb6/0x160
  ...

We observed the problem while experimenting with a touchscreen driver which
used MCP23017 IO expander (I2C).

The regmap in the pinctrl-mcp23s08 driver uses a mutex for protection from
concurrent accesses, which is the default for regmaps without .fast_io,
.disable_locking, etc.

mcp23s08_irq_set_type() calls regmap_update_bits_base(), and the latter
locks the mutex.

However, __setup_irq() locks desc->lock spinlock before calling these
functions. As a result, the system tries to lock the mutex whole holding
the spinlock.

It seems, the internal regmap locks are not needed in this driver at all.
mcp->lock seems to protect the regmap from concurrent accesses already,
except, probably, in mcp_pinconf_get/set.

mcp23s08_irq_set_type() and mcp23s08_irq_mask/unmask() are called under
chip_bus_lock(), which calls mcp23s08_irq_bus_lock(). The latter takes
mcp->lock and enables regmap caching, so that the potentially slow I2C
accesses are deferred until chip_bus_unlock().

The accesses to the regmap from mcp23s08_probe_one() do not need additional
locking.

In all remaining places where the regmap is accessed, except
mcp_pinconf_get/set(), the driver already takes mcp->lock.

This patch adds locking in mcp_pinconf_get/set() and disables internal
locking in the regmap config. Among other things, it fixes the sleeping
in atomic context described above.

Fixes: 8f38910 ("pinctrl: mcp23s08: switch to regmap caching")
Cc: stable@vger.kernel.org
Signed-off-by: Evgenii Shatokhin <e.shatokhin@yadro.com>
Link: https://lore.kernel.org/20241209074659.1442898-1-e.shatokhin@yadro.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Author: Evgenii Shatokhin

drivers/pinctrl/pinctrl-mcp23s08.c

@@ -86,6 +86,7 @@ const struct regmap_config mcp23x08_regmap = {
 	.num_reg_defaults = ARRAY_SIZE(mcp23x08_defaults),
 	.cache_type = REGCACHE_FLAT,
 	.max_register = MCP_OLAT,
+	.disable_locking = true, /* mcp->lock protects the regmap */
 };
 EXPORT_SYMBOL_GPL(mcp23x08_regmap);
 
@@ -132,6 +133,7 @@ const struct regmap_config mcp23x17_regmap = {
 	.num_reg_defaults = ARRAY_SIZE(mcp23x17_defaults),
 	.cache_type = REGCACHE_FLAT,
 	.val_format_endian = REGMAP_ENDIAN_LITTLE,
+	.disable_locking = true, /* mcp->lock protects the regmap */
 };
 EXPORT_SYMBOL_GPL(mcp23x17_regmap);
 
@@ -228,7 +230,9 @@ static int mcp_pinconf_get(struct pinctrl_dev *pctldev, unsigned int pin,
 
 	switch (param) {
 	case PIN_CONFIG_BIAS_PULL_UP:
+		mutex_lock(&mcp->lock);
 		ret = mcp_read(mcp, MCP_GPPU, &data);
+		mutex_unlock(&mcp->lock);
 		if (ret < 0)
 			return ret;
 		status = (data & BIT(pin)) ? 1 : 0;
@@ -257,7 +261,9 @@ static int mcp_pinconf_set(struct pinctrl_dev *pctldev, unsigned int pin,
 
 		switch (param) {
 		case PIN_CONFIG_BIAS_PULL_UP:
+			mutex_lock(&mcp->lock);
 			ret = mcp_set_bit(mcp, MCP_GPPU, pin, arg);
+			mutex_unlock(&mcp->lock);
 			break;
 		default:
 			dev_dbg(mcp->dev, "Invalid config param %04x\n", param);