Re: [RFC] libgpiod public API reviews needed

[Arnd Bergmann <arnd@arndb.de>]

On Mon, Jan 22, 2018 at 9:21 AM, Linus Walleij <linus.walleij@linaro.org> wrote:
> On Sun, Jan 21, 2018 at 11:18 PM, Arnd Bergmann <arnd@arndb.de> wrote:
>> On Sun, Jan 21, 2018 at 10:30 PM, Bartosz Golaszewski <brgl@bgdev.pl> wrote:
>>> 2018-01-21 16:49 GMT+01:00 Linus Walleij <linus.walleij@linaro.org>:
>>>> On Fri, Jan 19, 2018 at 2:28 PM, Bartosz Golaszewski <brgl@bgdev.pl> wrote:
>>
>>> I was not aware of this, but it seems you're right! Nice catch, thanks.
>>>
>>> How about defining a local struct gpiod_timespec with both seconds and
>>> nanoseconds explicitly defined to uint64_t?
>>
>> Where is that timestamp generated? Is this purely a user space interface
>> with the time read from gettimeofday(), or are we talking about  a new
>> kernel-to-user interface?
>
> This is in include/uapi/linux/gpio.h:
>
> /**
>  * struct gpioevent_data - The actual event being pushed to userspace
>  * @timestamp: best estimate of time of event occurrence, in nanoseconds
>  * @id: event identifier
>  */
> struct gpioevent_data {
>         __u64 timestamp;
>         __u32 id;
> };
>
> It is the same as is used for IIO. Inside the kernel this ultimately
> comes from ktime_get_real_ns();

Ah, too bad, that already contains two mistakes:

- on x86, the structures are incompatible between 32-bit and 64-bit
  user space, as the former has no padding.
- 'real' timestamps are inconvenient because time may jump in
  either direction. Time stamps should use 'monotonic' time, i.e.
  ktime_get_ns().

>> In a lot of cases, a simple 64-bit nanosecond counter using CLOCK_MONOTONIC
>> timestamps is the most robust and simple solution.
>
> Bartosz also seems to think it is the best so would vote to go
> for that and we have one problem less.

Could we introduce a new ioctl to replace the gpioevent_data() and
use a better interface then?

      Arnd