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

Grygorii Strashko grygorii.strashko at ti.com
Fri Mar 17 14:43:56 PDT 2017

On 03/17/2017 03:50 PM, David Rivshin wrote:
> On Fri, 17 Mar 2017 13:54:28 -0500
> Grygorii Strashko <grygorii.strashko at ti.com> wrote:
>> 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 at ti.com> wrote:
>>>> On 03/16/2017 07:57 PM, David Rivshin wrote:
>>>>> From: David Rivshin <DRivshin at 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 at vger.kernel.org> # 4.3+
>>>>> Signed-off-by: David Rivshin <drivshin at 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.
> I think that switching to software_debounce silently is exactly the
> intended/desired behavior of gpio-keys (and other drivers). For example,
> if the DT requests a 20ms debounce on a gpio-key, the existing math
> resulted in a hardware debounce of just 2ms. With the error return,
> gpio-keys would silently switch to software_debounce of the requested
> 20ms (potentially longer if the CPU is busy, but I don't think that's
> a problem for correctness), exactly what the DT asked for.
> Of course that would be a change in behavior for any such existing DT,
> and it's conceivable that the DT for some HW is somehow relying on that
> previous incorrect behavior. I suspect it's more likely that they are
> silently broken, and no-one has noticed. A quick search of some in-tree
> DTs finds most debounce times are 5ms (which has no issue), and then
> these three examples (all happen to be gpio-keys):
>   am335x-shc.dts:                 debounce-interval = <1000>;
>   am335x-shc.dts:                 debounce-interval = <1000>;
>   omap5-uevm.dts:                 debounce_interval = <50>;
> The first two currently result in a HW debounce of about 4ms. The
> third would be 2.5ms, except it's the wrong property name so it
> does nothing (it gets the default gpio-keys debounce of 5ms).

Yep. looks like error in dt. There are mod such DTs actually

> Not having seen any of that hardware, I can't say for certain what the
> true HW requirements are. 1000ms does seem like a long debounce, perhaps
> the author meant 1ms (1000us) for those buttons? Or perhaps it really
> needs a 1000ms debounce, and is currently wrong?
>> But agree - max might not be a good choose, so can you add dev_err() below, pls.
> Given the above, I personally feel that a dev_err() is undesirable in most
> cases. If I have a system and matching DT that just happens to need a longer
> debounce than the GPIO HW is capable of, gpio-keys (etc) does the best it can automatically. I don't consider that there is any error in that case, or
> anything to be fixed.
> I can understanding wanting to draw attention to a change in behavior (just
> in case the DT is incorrect), but I'd personally lean towards dev_info() if
> anything.
> That said: if you still prefer dev_err(), I will certainly do so.

Fair enough :) thanks.

Acked-by: Grygorii Strashko <grygorii.strashko at ti.com>

> Tangent:
> This discussion makes me think that adding a gpiod_get_max_debounce()
> would allow even better behavior. Then asking for a too-high debounce
> could be a dev_err() in all gpio drivers, with the expectation that no
> driver should ask for such. Also, drivers could do something like use
> max hardware debounce plus a software debounce for the remaining time,
> in order to avoid CPU overhead on short glitches.


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