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

[Grygorii Strashko <grygorii.strashko@ti.com>]

On 03/17/2017 12:54 PM, David Rivshin wrote:
> 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?

Yeh agree. But the problem here will be not only with drivers itself - it can be wrong data in DT :(
As result, even  gpio-keys driver will just silently switch to software_debounce
without any notification.

But agree - max might not be a good choose, so can you add dev_err() below, pls. 

> 
>>
>>>  	}
>>>  
>>>  	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);

if (ret) dev_err();

>>>  
>>> -	return 0;
>>> +	return ret;
>>>  }
>>>  
>>>  static int omap_gpio_set_config(struct gpio_chip *chip, unsigned offset,
>>>   
>>
> 
> 

-- 
regards,
-grygorii