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

On 03/17/2017 03:50 PM, David Rivshin wrote:
On Fri, 17 Mar 2017 13:54:28 -0500
Grygorii Strashko <grygorii.strashko@xxxxxx> 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@xxxxxx> wrote:

On 03/16/2017 07:57 PM, David Rivshin wrote:
From: David Rivshin <DRivshin@xxxxxxxxxxx>

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

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@xxxxxxxxxxxxxxx> # 4.3+
Signed-off-by: David Rivshin <drivshin@xxxxxxxxxxx>
 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;
+		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

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 =

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

That said: if you still prefer dev_err(), I will certainly do so.

Fair enough :) thanks.

Acked-by: Grygorii Strashko <grygorii.strashko@xxxxxx>

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.