Re: [PATCH 1/2] gpio: omap: return error if requested debounce time is not possible

[David Rivshin <drivshin@awxrd.com>]

Hi Grygorii,

On Fri, 17 Mar 2017 11:45:56 -0500
Grygorii Strashko <grygorii.strashko@ti.com> wrote:

> On 03/16/2017 07:57 PM, David Rivshin wrote:
> > From: David Rivshin <DRivshin@allworx.com>
> > 
> > omap_gpio_debounce() does not validate that the requested debounce
> > is within a range it can handle. Instead it lets the register value
> > wrap silently, and always returns success.
> > 
> > This can lead to all sorts of unexpected behavior, such as gpio_keys
> > asking for a too-long debounce, but getting a very short debounce in
> > practice.
> > 
> > Fix this by returning -EINVAL if the requested value does not fit into
> > the register field. If there is no debounce clock available at all,
> > return -ENOTSUPP.  
> 
> In general this patch looks good, but there is one thing I'm worry about..
> 
> > 
> > Fixes: e85ec6c3047b ("gpio: omap: fix omap2_set_gpio_debounce")
> > Cc: <stable@vger.kernel.org> # 4.3+
> > Signed-off-by: David Rivshin <drivshin@allworx.com>
> > ---
> >  drivers/gpio/gpio-omap.c | 16 +++++++++++-----
> >  1 file changed, 11 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/gpio/gpio-omap.c b/drivers/gpio/gpio-omap.c
> > index efc85a2..33ec02d 100644
> > --- a/drivers/gpio/gpio-omap.c
> > +++ b/drivers/gpio/gpio-omap.c
> > @@ -208,8 +208,10 @@ static inline void omap_gpio_dbck_disable(struct gpio_bank *bank)
> >   * OMAP's debounce time is in 31us steps
> >   *   <debounce time> = (GPIO_DEBOUNCINGTIME[7:0].DEBOUNCETIME + 1) x 31
> >   * so we need to convert and round up to the closest unit.
> > + *
> > + * Return: 0 on success, negative error otherwise.
> >   */
> > -static void omap2_set_gpio_debounce(struct gpio_bank *bank, unsigned offset,
> > +static int omap2_set_gpio_debounce(struct gpio_bank *bank, unsigned offset,
> >  				    unsigned debounce)
> >  {
> >  	void __iomem		*reg;
> > @@ -218,11 +220,12 @@ static void omap2_set_gpio_debounce(struct gpio_bank *bank, unsigned offset,
> >  	bool			enable = !!debounce;
> >  
> >  	if (!bank->dbck_flag)
> > -		return;
> > +		return -ENOTSUPP;
> >  
> >  	if (enable) {
> >  		debounce = DIV_ROUND_UP(debounce, 31) - 1;
> > -		debounce &= OMAP4_GPIO_DEBOUNCINGTIME_MASK;
> > +		if ((debounce & OMAP4_GPIO_DEBOUNCINGTIME_MASK) != debounce)
> > +			return -EINVAL;  
> 
> This might cause boot issues as current drivers may expect this op to succeed even if
> configured value is wrong - just think, may be we can do warn here and use max value as
> fallback?

I have not looked through all drivers to be sure, but at least the gpio-keys 
driver requires set_debounce to return an error if it can't satisfy the request. 
In that case gpio-keys will use a software timer instead. 

                if (button->debounce_interval) {
                        error = gpiod_set_debounce(bdata->gpiod,
                                        button->debounce_interval * 1000);
                        /* use timer if gpiolib doesn't provide debounce */
                        if (error < 0)
                                bdata->software_debounce =
                                                button->debounce_interval;
                }

Also, at least some other GPIO drivers (e.g. gpio-max7760) return -EINVAL in 
such a case. And gpiolib will return -ENOTSUPP if there is no debounce 
callback at all. So I expect all drivers which use gpiod_set_debounce() to 
handle error returns gracefully. 

So I certainly understand the concern about backwards compatibility, but I 
think clipping to max is the greater of the evils in this case. Even a 
warning may be too much, because it's not necessarily anything wrong. 
Perhaps an info or debug message would be helpful, though?

If you prefer, I can try to go through all callers of gpiod_set_debounce()
and see how they'd handle an error return. The handful I've looked through so 
far all behave like gpio-keys. The only ones I'd be particularly concerned
about are platform-specific drivers which were perhaps never used with other
gpio drivers. Do you know of that I should pay special attention to?

> 
> >  	}
> >  
> >  	l = BIT(offset);
> > @@ -255,6 +258,8 @@ static void omap2_set_gpio_debounce(struct gpio_bank *bank, unsigned offset,
> >  		bank->context.debounce = debounce;
> >  		bank->context.debounce_en = val;
> >  	}
> > +
> > +	return 0;
> >  }
> >  
> >  /**
> > @@ -964,14 +969,15 @@ static int omap_gpio_debounce(struct gpio_chip *chip, unsigned offset,
> >  {
> >  	struct gpio_bank *bank;
> >  	unsigned long flags;
> > +	int ret;
> >  
> >  	bank = gpiochip_get_data(chip);
> >  
> >  	raw_spin_lock_irqsave(&bank->lock, flags);
> > -	omap2_set_gpio_debounce(bank, offset, debounce);
> > +	ret = omap2_set_gpio_debounce(bank, offset, debounce);
> >  	raw_spin_unlock_irqrestore(&bank->lock, flags);
> >  
> > -	return 0;
> > +	return ret;
> >  }
> >  
> >  static int omap_gpio_set_config(struct gpio_chip *chip, unsigned offset,
> >   
>