[PATCH 02/12] drivers/gpio: provide optional timestamped readouts

Jan Kiszka jan.kiszka at siemens.com
Fri Jan 25 12:32:16 CET 2019


On 25.01.19 11:11, Philippe Gerum wrote:
> On 1/25/19 10:48 AM, Jan Kiszka wrote:
>> On 25.01.19 10:38, Philippe Gerum wrote:
>>> On 1/25/19 10:33 AM, Jan Kiszka wrote:
>>>> On 25.01.19 10:31, Philippe Gerum wrote:
>>>>> On 1/25/19 10:23 AM, Jan Kiszka wrote:
>>>>>> On 25.01.19 10:15, Philippe Gerum wrote:
>>>>>>> On 1/24/19 7:17 PM, Jan Kiszka wrote:
>>>>>>>> On 24.01.19 16:34, Philippe Gerum wrote:
>>>>>>>>> In timestamping mode, read() returns the timestamp of the latest
>>>>>>>>> event
>>>>>>>>> receipt on the pin based on CLOCK_MONOTONIC, along with the pin
>>>>>>>>> state. This is an optional pin readout mode controlled by the
>>>>>>>>> GPIO_RTIOC_TS request, e.g.:
>>>>>>>>>
>>>>>>>>> struct rtdm_gpio_readout rdo;
>>>>>>>>> int ret, on, val;
>>>>>>>>>
>>>>>>>>> on = 1;
>>>>>>>>> ret = ioctl(pinfd, GPIO_RTIOC_TS, &on);
>>>>>>>>> ret = read(pinfd, &rdo, sizeof(rdo));
>>>>>>>>> /* pin state changed to rdo.value at time rdo.timestamp */
>>>>>>>>>
>>>>>>>>> on = 0;
>>>>>>>>> ret = ioctl(pinfd, GPIO_RTIOC_TS, &on);
>>>>>>>>> ret = read(pinfd, &val, sizeof(val));
>>>>>>>>> /* pin state changed to value (time of change unspecified) */
>>>>>>>>>
>>>>>>>>> By default, timestamping mode is disabled, which corresponds to the
>>>>>>>>> original behavior.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Philippe Gerum <rpm at xenomai.org>
>>>>>>>>> ---
>>>>>>>>>       include/cobalt/kernel/rtdm/gpio.h |  1 +
>>>>>>>>>       include/rtdm/uapi/gpio.h          | 18 +++++++----
>>>>>>>>>       kernel/drivers/gpio/gpio-core.c   | 54
>>>>>>>>> +++++++++++++++++++++++--------
>>>>>>>>>       3 files changed, 54 insertions(+), 19 deletions(-)
>>>>>>>>>
>>>>>>>>> diff --git a/include/cobalt/kernel/rtdm/gpio.h
>>>>>>>>> b/include/cobalt/kernel/rtdm/gpio.h
>>>>>>>>> index cdb472f8a..00055ec0a 100644
>>>>>>>>> --- a/include/cobalt/kernel/rtdm/gpio.h
>>>>>>>>> +++ b/include/cobalt/kernel/rtdm/gpio.h
>>>>>>>>> @@ -33,6 +33,7 @@ struct rtdm_gpio_pin {
>>>>>>>>>           rtdm_event_t event;
>>>>>>>>>           char *name;
>>>>>>>>>           struct gpio_desc *desc;
>>>>>>>>> +    nanosecs_abs_t timestamp;
>>>>>>>>>       };
>>>>>>>>>         struct rtdm_gpio_chip {
>>>>>>>>> diff --git a/include/rtdm/uapi/gpio.h b/include/rtdm/uapi/gpio.h
>>>>>>>>> index b745f156c..ac14be66c 100644
>>>>>>>>> --- a/include/rtdm/uapi/gpio.h
>>>>>>>>> +++ b/include/rtdm/uapi/gpio.h
>>>>>>>>> @@ -18,12 +18,18 @@
>>>>>>>>>       #ifndef _RTDM_UAPI_GPIO_H
>>>>>>>>>       #define _RTDM_UAPI_GPIO_H
>>>>>>>>>       -#define GPIO_RTIOC_DIR_OUT        _IOW(RTDM_CLASS_GPIO, 0,
>>>>>>>>> int)
>>>>>>>>> -#define GPIO_RTIOC_DIR_IN        _IO(RTDM_CLASS_GPIO, 1)
>>>>>>>>> -#define GPIO_RTIOC_IRQEN        _IOW(RTDM_CLASS_GPIO, 2, int) /*
>>>>>>>>> GPIO
>>>>>>>>> trigger */
>>>>>>>>> -#define GPIO_RTIOC_IRQDIS        _IO(RTDM_CLASS_GPIO, 3)
>>>>>>>>> -#define GPIO_RTIOC_REQS                _IO(RTDM_CLASS_GPIO, 4)
>>>>>>>>> -#define GPIO_RTIOC_RELS                _IO(RTDM_CLASS_GPIO, 5)
>>>>>>>>> +struct rtdm_gpio_readout {
>>>>>>>>> +    __u64 timestamp;
>>>>>>>>
>>>>>>>> nanosecs_abs_t - we use this type also in to userspace interface.
>>>>>>>>
>>>>>>>
>>>>>>> Nope. This is ABI stuff, nanosecs_abs_t is not guaranteed to have the
>>>>>>> same size regardless of the ABI models, which is unsafe in mixed
>>>>>>> 32/64
>>>>>>> model configurations. OTOH, __u64 is safe.
>>>>>>
>>>>>> If nanosecs_abs_t is alread an ABI, and it is used as such for a long
>>>>>> time (see include/rtdm/uapi/serial.h). If you see a bug in its
>>>>>> definition, that needs to be fixed. We should not resolve that by
>>>>>> choosing open-coded local workarounds.
>>>>>>
>>>>>
>>>>> nanosecs_t is not defined in the Xenomai ABI, never has been. Using
>>>>> nanosecs_t in structs actually abuses such ABI. So we may either keep
>>>>> adding even more nanosecs, or fix the spots where it should be replaced
>>>>> with __u64.
>>>>
>>>> Xenomai ABI includes the driver ABI - so your assumption might have been
>>>> it's not while it was de facto.
>>>>
>>>> Again, let's address the issue, not work around it.
>>>>
>>>
>>> __u64 is the fix, using nanosecs*_t in uapi/ is wrong. So please merge
>>> this fix, and we'll see how we can progress fixing other abuses.
>>
>> Nope: Adjust nanosecs_*_t if it has a problem ([u]int64_t vs. __u/s64 -
>> what's the problem exactly?), and use that. That will both address
>> existing users and future ones. No special solution for the GPIO driver
>> here.
>>
> 
> I'm not pushing for a GPIO-specific solution, I'm pushing for using
> ABI-specific types which state their size explicitly in uapi/ data as
> much as possible, which is safer with mixed ABI models and would have
> spared me a lot of trouble back when I implemented the 32/64 ABI support
> for Xenomai. nanosecs_* is originally a kernel-side type which slipped
> in uapi/ structs, which does not state its size.

No, it was designed and used for both side. I know best as I wrote and first 
used that abstraction.

> 
> The fact that nanosecs_* is defined as uint64_t but the kernel today
> does not address the issue at stake: there would be no obvious way to
> detect that a discrepancy might exist with what userland expects once
> such definition is modified for a different with if ever (which might be
> highly unlikely for nanosecs, but could happen for other types).

So the good news is that a) we do not have a problem today and b) we can safely 
prevent any hypothetical future by changing the definition of nanosecs_*_t 
towards __u64/__s64 backing. So, if you are concerned about the stability of 
standard-int, let's switch to the kernel uapi types for nanosecs, use that also 
for the GPIO ABI, and call it a day.

Jan

-- 
Siemens AG, Corporate Technology, CT RDA IOT SES-DE
Corporate Competence Center Embedded Linux



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