linux-kernel - Re: [PATCH v5 2/3] input: evdev: Replace timeval with timespec64

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Date:   Sat, 6 Jan 2018 13:43:34 -0800
From:   Deepa Dinamani <deepa.kernel@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        "open list:HID CORE LAYER" <linux-input@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Hutterer <peter.hutterer@...-t.net>,
        y2038 Mailman List <y2038@...ts.linaro.org>
Subject: Re: [PATCH v5 2/3] input: evdev: Replace timeval with timespec64

On Tue, Jan 2, 2018 at 7:35 AM, Arnd Bergmann <arnd@...db.de> wrote:
> On Tue, Jan 2, 2018 at 7:46 AM, Dmitry Torokhov
> <dmitry.torokhov@...il.com> wrote:
>> On Sun, Dec 17, 2017 at 09:18:43PM -0800, Deepa Dinamani wrote:
>>> @@ -304,12 +314,11 @@ static void evdev_events(struct input_handle *handle,
>>>  {
>>>       struct evdev *evdev = handle->private;
>>>       struct evdev_client *client;
>>> -     ktime_t ev_time[EV_CLK_MAX];
>>> +     struct timespec64 ev_time[EV_CLK_MAX];
>>>
>>> -     ev_time[EV_CLK_MONO] = ktime_get();
>>> -     ev_time[EV_CLK_REAL] = ktime_mono_to_real(ev_time[EV_CLK_MONO]);
>>> -     ev_time[EV_CLK_BOOT] = ktime_mono_to_any(ev_time[EV_CLK_MONO],
>>> -                                              TK_OFFS_BOOT);
>>> +     ktime_get_ts64(&ev_time[EV_CLK_MONO]);
>>> +     ktime_get_real_ts64(&ev_time[EV_CLK_REAL]);
>>> +     get_monotonic_boottime64(&ev_time[EV_CLK_BOOT]);
>>
>> This may result in different ev_time[] members holding different times,
>> whereas the original code would take one time sample and convert it to
>> different clocks.
>
> Is this important? On each client we only return one of the two
> times, and I would guess that you cannot rely on a correlation
> between timestamps on different devices, since the boot and real
> offsets can change over time.

Right. I didn't think this was an issue either.

>> Also, why can't we keep using ktime_t internally? It is y2038 safe,
>> right?
>
> Correct, but there may also be a performance difference if we get
> a lot of events, not sure if that matters.
>
>> I think you should drop this patch and adjust the 3rd one to
>> massage the input event timestamp patch to do ktime->timespec64->input
>> timestamp conversion.
>
> The change in __evdev_queue_syn_dropped still seems useful to me
> as ktime_get_*ts64() is a bit more efficient than ktime_get*() followed by
> a slow ktime_to_timespec64() or ktime_to_timeval().
>
> For evdev_events(), doing a single ktime_get() followed by a
> ktime_to_timespec64/ktime_to_timeval can be faster than three
> ktime_get_*ts64 (depending on the hardware clock source), or
> it can be slower depending on the CPU and the clocksource
> hardware. Again, no idea if this matters at the usual rate of
> input events.
>
> I guess dropping the evdev_events() change and replacing it with a
> ktime_to_timespec64 change in evdev_pass_values()
> would be fine here, it should keep the current performance
> behavior and get rid of the timeval.

I was trying to use timespec64 everywhere so that we would not have
conversions back and forth at the input layer.
I dropped the ktime_t conversions for now and merged this patch with
the next one as requested.

Let me know if you would like to keep the changes Arnd preferred above
for __evdev_queue_syn_dropped(). I can submit a separate patch if this
is preferred.

-Deepa