gpio: omap: Silence lockdep "Invalid wait context"

From: Alexander Sverdlin <alexander.sverdlin@siemens.com>

The problem apparetly has been known since the conversion to
raw_spin_lock() (commit 4dbada2be460
("gpio: omap: use raw locks for locking")).

Symptom:

[ BUG: Invalid wait context ]
5.10.214
-----------------------------
swapper/1 is trying to lock:
(enable_lock){....}-{3:3}, at: clk_enable_lock
other info that might help us debug this:
context-{5:5}
2 locks held by swapper/1:
 #0: (&dev->mutex){....}-{4:4}, at: device_driver_attach
 #1: (&bank->lock){....}-{2:2}, at: omap_gpio_set_config
stack backtrace:
CPU: 0 PID: 1 Comm: swapper Not tainted 5.10.214
Hardware name: Generic AM33XX (Flattened Device Tree)
unwind_backtrace
show_stack
__lock_acquire
lock_acquire.part.0
_raw_spin_lock_irqsave
clk_enable_lock
clk_enable
omap_gpio_set_config
gpio_keys_setup_key
gpio_keys_probe
platform_drv_probe
really_probe
driver_probe_device
device_driver_attach
__driver_attach
bus_for_each_dev
bus_add_driver
driver_register
do_one_initcall
do_initcalls
kernel_init_freeable
kernel_init

Problematic spin_lock_irqsave(&enable_lock, ...) is being called by
clk_enable()/clk_disable() in omap2_set_gpio_debounce() and
omap_clear_gpio_debounce().

For omap2_set_gpio_debounce() it's possible to move
raw_spin_lock_irqsave(&bank->lock, ...) inside omap2_set_gpio_debounce()
so that the locks nest as follows:

  clk_enable(bank->dbck)
  raw_spin_lock_irqsave(&bank->lock, ...)
  raw_spin_unlock_irqrestore()
  clk_disable()

Two call-sites of omap_clear_gpio_debounce() are more convoluted, but one
can take the advantage of the nesting nature of clk_enable()/clk_disable(),
so that the inner clk_disable() becomes lockless:

  clk_enable(bank->dbck)		<-- only to clk_enable_lock()
  raw_spin_lock_irqsave(&bank->lock, ...)
  omap_clear_gpio_debounce()
    clk_disable()			<-- becomes lockless
  raw_spin_unlock_irqrestore()
  clk_disable()

Cc: stable@vger.kernel.org
Fixes: 4dbada2be460 ("gpio: omap: use raw locks for locking")
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
---
 drivers/gpio/gpio-omap.c | 35 ++++++++++++++++++++++++++++++-----
 1 file changed, 30 insertions(+), 5 deletions(-)

Thanks Bartosz for looking into it!

On Wed, 2024-11-27 at 11:04 +0100, Bartosz Golaszewski wrote:
> > > > @@ -827,6 +846,13 @@ static void omap_gpio_free(struct gpio_chip *chip, unsigned offset)
> > > >           struct gpio_bank *bank = gpiochip_get_data(chip);
> > > >           unsigned long flags;
> > > > 
> > > > +       /*
> > > > +        * Enable the clock here so that the nested clk_disable() in the
> > > > +        * following omap_clear_gpio_debounce() is lockless
> > > > +        */
> > > > +       if (bank->dbck_flag)
> > > > +               clk_enable(bank->dbck);
> >                      ^^^^^^^^^^^^^^^^^^^^^^
> > And for this second enable....
> > 
> > > > +
> > > >           raw_spin_lock_irqsave(&bank->lock, flags);
> > > >           bank->mod_usage &= ~(BIT(offset));
> > > >           if (!LINE_USED(bank->irq_usage, offset)) {
> > > > @@ -836,6 +862,9 @@ static void omap_gpio_free(struct gpio_chip *chip, unsigned offset)
> > > >           omap_disable_gpio_module(bank, offset);
> > > >           raw_spin_unlock_irqrestore(&bank->lock, flags);
> > > > 
> > > > +       if (bank->dbck_flag)
> > > > +               clk_disable(bank->dbck);
> >                      ^^^^^^^^^^^^^^^^^^^^^^^
> > ... this would be the corresponding 2nd disable.
> > 
> > > > +
> > > >           pm_runtime_put(chip->parent);
> > > >    }
> > > > 
> > 
> > Aren't they paired from your PoV?
> > 
> 
> Ok, I needed to take a long look at this driver.
> 
> IIUC what happens now:
> 
> In omap_gpio_irq_shutdown() and omap_gpio_free() you now enable the
> clock (really just bumping its enable count) before entering
> omap_clear_gpio_debounce() so that its internal call do clk_disable()
> only decreases the reference count without actually disabling it and
> the clock is really disabled one level up the stack.
> 
> If that's correct, isn't it unnecessarily convoluted?
> omap_clear_gpio_debounce() is only called from these two functions so
> why not just move the clk_disable() out of it and into them?

Seems that I didn't notice the elephant in the room, so let me rework it ;-)