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

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

On 04/20/2017 09:44 AM, David Rivshin wrote:
> Hi Grygorii,
>
> Not sure if you saw the question at the bottom asking for clarification
> on what you'd prefer as far as any dev_xxx() message for this case. If
> there is still concern on the other patch, I could just resubmit this
> standalone (perhaps aiming for 4.12 at this point).

Could you add dev info and resubmit this alone, pls


>
> On Fri, 17 Mar 2017 19:42:35 -0400
> David Rivshin <drivshin@awxrd.com> wrote:
>
>> On Fri, 17 Mar 2017 16:43:56 -0500
>> Grygorii Strashko <grygorii.strashko@ti.com> wrote:
>>
>>> On 03/17/2017 03:50 PM, David Rivshin wrote:
>>>> On Fri, 17 Mar 2017 13:54:28 -0500
>>>> Grygorii Strashko <grygorii.strashko@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@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.
>>>>
>>>> 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.
>>>>
> [...snip...]
>>>>>
>>>>> 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@ti.com>
>>
>> Just to make sure I don't misunderstand, would you like me to:
>> A) put in a dev_err()
>> B) put in a dev_info()
>> C) leave it as-is without any message
>> ?
>>
> [...snip...]
>
> FYI, I have searched for all uses of gpio{,d}_set_debounce (in v4.11-rc1),
> and found nothing concerning. Most drivers fall back to software debounce.
>
> The only exception I found was mmc_spi (via mmc_gpio_request_cd), but the
> only time that has a non-zero debounce requested is for vision_ep9307 which
> is hardcoded to ask for a 1us debounce via platform data. I don't believe
> ep93xx would use the gpio-omap driver anyways. The mmc-spi-slot devicetree
> binding doesn't support setting a debounce on any of the GPIOs.
>

-- 
regards,
-grygorii