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

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.

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