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

In the Linux kernel, the following vulnerability has been resolved:

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.