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From: Jonathan Cameron <jic23@kernel.org>
To: Lars-Peter Clausen <lars@metafoo.de>,
	Gregor Boirie <gregor.boirie@parrot.com>,
	Jonathan Cameron <jic23@jic23.retrosnub.co.uk>,
	linux-iio@vger.kernel.org
Subject: Re: using monotonic clok for timstamping
Date: Tue, 9 Feb 2016 21:02:04 +0000	[thread overview]
Message-ID: <56BA53CC.40808@kernel.org> (raw)
In-Reply-To: <56BA5176.6020108@metafoo.de>

On 09/02/16 20:52, Lars-Peter Clausen wrote:
> On 02/09/2016 12:06 PM, Gregor Boirie wrote:
>> To sump up, implementing timestamping clock selection support in IIO
>> should be based on the following principles.
>>
>> Selected timestamping clock is a per-device attribute which the userspace
>> may access through a sysfs file.
>>
>> It must be applicable to both buffered samples and events.
> 
> Just playing out ideas here to make sure we have everything covered, but
> could be there be a setup where you'd want different timestamps for events
> and buffers?
I hope not..
> 
>>
>> Userspace may choose amongst a subset of available POSIX clocks. A good
>> starting point would be: CLOCK_REALTIME, CLOCK_MONOTONIC,
>> CLOCK_MONOTONIC_RAW, CLOCK_REALTIME_COARSE,
>> CLOCK_MONOTONIC_COARSE, CLOCK_BOOTTIME and CLOCK_TAI.
>> Please delete as appropriate if needed and see clock_gettime(2).
>>
>> One remaining question is : how do we ensure sample and event timestamps
>> consistency with respect to clock changes ? 2 suggestions:
>> * reject the ability to select a new clock as long as an events chardev is
>> opened
>>   or a buffered samples channel is enabled ;
>> * clock may be changed at any time since it could be implemented in an atomic
>>   way (a simple atomic_t can hold an int / clockid_t if I'm no wrong).
> 
> Not sure if we need to prevent it. If somebody changes it during an active
> measurement it is their problem I'd say.
A fair point if we document it clearly as 'HERE BE DRAGONS!'
Won't typically crash anything, just give crazy answers :)
> 


  reply	other threads:[~2016-02-09 21:02 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-02-03 10:55 Gregor Boirie
2016-02-03 11:22 ` Lars-Peter Clausen
2016-02-06 18:33   ` Jonathan Cameron
2016-02-08  9:55     ` Lars-Peter Clausen
2016-02-08 17:15       ` Jonathan Cameron
2016-02-09 11:06         ` Gregor Boirie
2016-02-09 14:19           ` Gregor Boirie
2016-02-09 20:49             ` Jonathan Cameron
2016-02-09 20:52           ` Lars-Peter Clausen
2016-02-09 21:02             ` Jonathan Cameron [this message]
2016-02-10  9:46             ` Gregor Boirie

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