Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751332AbdCRBTG (ORCPT ); Fri, 17 Mar 2017 21:19:06 -0400 Received: from lelnx193.ext.ti.com ([198.47.27.77]:30552 "EHLO lelnx193.ext.ti.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751023AbdCRBTE (ORCPT ); Fri, 17 Mar 2017 21:19:04 -0400 Subject: Re: [PATCH 1/2] gpio: omap: return error if requested debounce time is not possible To: David Rivshin References: <20170317005704.11971-1-drivshin@awxrd.com> <20170317005704.11971-2-drivshin@awxrd.com> <20170317135413.78118dc2.drivshin@awxrd.com> <20170317165032.5b891bc1.drivshin@awxrd.com> CC: , , Santosh Shilimkar , Kevin Hilman , Linus Walleij , Alexandre Courbot , , , From: Grygorii Strashko Message-ID: <73fd00d4-6649-90d6-dcb1-0cd186055a69@ti.com> Date: Fri, 17 Mar 2017 16:43:56 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0 MIME-Version: 1.0 In-Reply-To: <20170317165032.5b891bc1.drivshin@awxrd.com> Content-Type: text/plain; charset="windows-1252"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [128.247.83.96] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 7736 Lines: 171 On 03/17/2017 03:50 PM, David Rivshin wrote: > On Fri, 17 Mar 2017 13:54:28 -0500 > Grygorii Strashko wrote: > >> On 03/17/2017 12:54 PM, David Rivshin wrote: >>> Hi Grygorii, >>> >>> On Fri, 17 Mar 2017 11:45:56 -0500 >>> Grygorii Strashko wrote: >>> >>>> On 03/16/2017 07:57 PM, David Rivshin wrote: >>>>> From: David Rivshin >>>>> >>>>> 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: # 4.3+ >>>>> Signed-off-by: David Rivshin >>>>> --- >>>>> 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 >>>>> * = (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 > > > 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