linux-kernel - Re: [PATCH 001/001] [Input:] Propagate hardware generated event timestamp to evdev.

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Message-ID: <CAJVTVrmf9peeM=D=GscUp-fNDRPwkuhbxnkbvYN+i+n+PE=PzQ@mail.gmail.com>
Date:	Wed, 17 Jul 2013 13:31:06 -0700
From:	Alexander Levitskiy <sanek@...gle.com>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, Rob Landley <rob@...dley.net>,
	Henrik Rydberg <rydberg@...omail.se>,
	Arve Hjønnevåg <arve@...roid.com>,
	Stephen Boyd <sboyd@...eaurora.org>
Subject: Re: [PATCH 001/001] [Input:] Propagate hardware generated event
 timestamp to evdev.

Dmitry,

I didn't see the MSC_TIMESTAMP before. I should have been more up to
date. I've looked at MSC_TIMESTAMP change and the first impression was
that it is something we can live with. After discussing details with
my colleagues here we came to a conclusion that:
a) we do need absolute time.
b) since we have to use 64-bit timestamp there is no point to discard
extra precision especially in light of event timestamp being of ns
precision.
c) there is no point to propagate the timestamp to userspace since
there is an existing mechanism in place.

I think the concept of propagating hardware timestamp in ABS_MT fits
nicely in the "simultaneous contact" model of multitouch operation.
I'd like to keep this functionality.

Thank you,
- Sasha.

On Fri, Jul 12, 2013 at 11:44 PM, Dmitry Torokhov
<dmitry.torokhov@...il.com> wrote:
> Hi Sasha,
>
> On Wed, Jul 10, 2013 at 01:38:00PM -0700, Alexander Levitskiy wrote:
>> From: Sasha Levitskiy <sanek@...gle.com>
>>
>> Input: Propagate hardware event timestamp to evdev.
>>
>> Convey hardware generated timestamp associated with the current event packet.
>> The use of these event codes by hardware drivers is optional.
>> Used to reduce jitter and improve velocity tracking in ABS_MT and other timing
>> sensitive devices.
>>
>> kernel v. 3.4
>>
>> Signed-off-by: Sasha Levitskiy <sanek@...gle.com>
>> ---
>> diff --git a/Documentation/input/event-codes.txt
>> b/Documentation/input/event-codes.txt
>> index 53305bd..f0f0e07 100644
>> --- a/Documentation/input/event-codes.txt
>> +++ b/Documentation/input/event-codes.txt
>> @@ -91,6 +91,15 @@ sent in the evdev event stream.
>>      event and query the device (using EVIOCG* ioctls) to obtain its
>>      current state.
>>
>> +* SYN_TIME_SEC, SYN_TIME_NSEC:
>> +  - Used to convey hardware timestamp associated with the current
>> +    event packet. The use of these event codes by hardware drivers
>> +    is optional. If used, the hardware driver should send the timestamp
>> +    ahead of any other events associated with this packet. The timestamp
>> +    should be adjusted to CLOCK_MONOTONIC base.
>> +    This becomes useful for drivers of hardware that handle batching
>> +    without involving the main CPU.
>
> We already have MSC_TIMESTAMP (which is in usec), do you really need
> nsec resolution?
>
>> +
>>  EV_KEY:
>>  ----------
>>  EV_KEY events take the form KEY_<name> or BTN_<name>. For example,
>> KEY_A is used
>> diff --git a/Documentation/input/multi-touch-protocol.txt
>> b/Documentation/input/multi-touch-protocol.txt
>> index 543101c..71af317 100644
>> --- a/Documentation/input/multi-touch-protocol.txt
>> +++ b/Documentation/input/multi-touch-protocol.txt
>> @@ -80,6 +80,10 @@ Userspace can detect that a driver can report more
>> total contacts than slots
>>  by noting that the largest supported BTN_TOOL_*TAP event is larger than the
>>  total number of type B slots reported in the absinfo for the ABS_MT_SLOT axis.
>>
>> +Velocity tracking and temporal precision can be improved if device provides
>> +exact timestamp for touches reported through SYN_TIME_SEC and SYN_TIME_NSEC.
>> +The timestamp should be reported ahead of everything else in the packet.
>> +
>>  Protocol Example A
>>  ------------------
>>
>> diff --git a/drivers/input/evdev.c b/drivers/input/evdev.c
>> index 41f79be..48baf6f 100644
>> --- a/drivers/input/evdev.c
>> +++ b/drivers/input/evdev.c
>> @@ -37,6 +37,8 @@ struct evdev {
>>   struct mutex mutex;
>>   struct device dev;
>>   bool exist;
>> + int hw_ts_sec;
>> + int hw_ts_nsec;
>>  };
>>
>>  struct evdev_client {
>> @@ -109,7 +111,20 @@ static void evdev_event(struct input_handle *handle,
>>   struct input_event event;
>>   ktime_t time_mono, time_real;
>>
>> - time_mono = ktime_get();
>> + if (type == EV_SYN && code == SYN_TIME_SEC) {
>> + evdev->hw_ts_sec = value;
>> + return;
>
> Why do you need this special handling? Can you simply have MSC_TIMESTAMP
> to be delivered as part of the event packet and use it instead of the
> event timestamp if userspace chooses to do so?
>
> Thanks.
>
> --
> Dmitry
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