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

Started byDavid Rivshin <drivshin@awxrd.com>
First post2017-03-17 19:40 +0100
Last post2017-03-18 02:20 +0100
Articles 5 — 2 participants

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  Re: [PATCH 1/2] gpio: omap: return error if requested debounce time  is not possible David Rivshin <drivshin@awxrd.com> - 2017-03-17 19:40 +0100
    Re: [PATCH 1/2] gpio: omap: return error if requested debounce time  is not possible Grygorii Strashko <grygorii.strashko@ti.com> - 2017-03-17 20:00 +0100
      Re: [PATCH 1/2] gpio: omap: return error if requested debounce time  is not possible David Rivshin <drivshin@awxrd.com> - 2017-03-17 22:10 +0100
        Re: [PATCH 1/2] gpio: omap: return error if requested debounce time  is not possible David Rivshin <drivshin@awxrd.com> - 2017-03-18 00:50 +0100
        Re: [PATCH 1/2] gpio: omap: return error if requested debounce time  is not possible Grygorii Strashko <grygorii.strashko@ti.com> - 2017-03-18 02:20 +0100

#1603538 — Re: [PATCH 1/2] gpio: omap: return error if requested debounce time is not possible

FromDavid Rivshin <drivshin@awxrd.com>
Date2017-03-17 19:40 +0100
SubjectRe: [PATCH 1/2] gpio: omap: return error if requested debounce time is not possible
Message-ID<tm4X7-2fG-1@gated-at.bofh.it>
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,
> >   
>

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#1603568

FromGrygorii Strashko <grygorii.strashko@ti.com>
Date2017-03-17 20:00 +0100
Message-ID<tm5gv-2nQ-55@gated-at.bofh.it>
In reply to#1603538

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

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#1603624

FromDavid Rivshin <drivshin@awxrd.com>
Date2017-03-17 22:10 +0100
Message-ID<tm7ii-45y-11@gated-at.bofh.it>
In reply to#1603568
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.

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).

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.


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. 

> >>  
> >>>  	}
> >>>  
> >>>  	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,
> >>>     
> >>  
> > 
> >   
>

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#1603664

FromDavid Rivshin <drivshin@awxrd.com>
Date2017-03-18 00:50 +0100
Message-ID<tm9N8-5Uu-5@gated-at.bofh.it>
In reply to#1603624
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.
> >
> > 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
> ./arch/arm/boot/dts/atlas7-evb.dts
> ./arch/arm/boot/dts/emev2-kzm9d.dts
> ./arch/arm/boot/dts/kirkwood-pogoplug-series-4.dts
> ./arch/arm/boot/dts/omap5-uevm.dts
> ./arch/arm/boot/dts/ste-snowball.dts

Ah yes, I just grepped for 'debounce' in am* and omap*. I guess
that typo has been copied over from DT to DT. I'm tempted to spin 
a patch correcting the typo, but I have no knowledge of those
boards or HW to test with. Obviously no-one has complained about 
the 5ms vs 50ms debounce so far, so maybe 50ms isn't the correct
number in the first place?

> >
> > 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@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 
?

I can spin a v2 as early as Monday, depending on the results of discussion 
on the second patch.

> 
> >
> > 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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#1603701

FromGrygorii Strashko <grygorii.strashko@ti.com>
Date2017-03-18 02:20 +0100
Message-ID<tm9N8-5Uu-7@gated-at.bofh.it>
In reply to#1603624

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.
>
> 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
./arch/arm/boot/dts/atlas7-evb.dts
./arch/arm/boot/dts/emev2-kzm9d.dts
./arch/arm/boot/dts/kirkwood-pogoplug-series-4.dts
./arch/arm/boot/dts/omap5-uevm.dts
./arch/arm/boot/dts/ste-snowball.dts


>
> 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@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.
>
>

-- 
regards,
-grygorii

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