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Message-ID: <20170317165032.5b891bc1.drivshin@awxrd.com>
Date: Fri, 17 Mar 2017 16:50:32 -0400
From: David Rivshin <drivshin@...rd.com>
To: Grygorii Strashko <grygorii.strashko@...com>
Cc: <linux-gpio@...r.kernel.org>, <linux-omap@...r.kernel.org>,
Santosh Shilimkar <ssantosh@...nel.org>,
Kevin Hilman <khilman@...nel.org>,
Linus Walleij <linus.walleij@...aro.org>,
Alexandre Courbot <gnurou@...il.com>,
<linux-arm-kernel@...ts.infradead.org>,
<linux-kernel@...r.kernel.org>, <stable@...r.kernel.org>
Subject: Re: [PATCH 1/2] gpio: omap: return error if requested debounce time
is not possible
On Fri, 17 Mar 2017 13:54:28 -0500
Grygorii Strashko <grygorii.strashko@...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@...com> wrote:
> >
> >> On 03/16/2017 07:57 PM, David Rivshin wrote:
> >>> From: David Rivshin <DRivshin@...worx.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@...r.kernel.org> # 4.3+
> >>> Signed-off-by: David Rivshin <drivshin@...worx.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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