Re: About timestamping and can-utils

[Stephane Grosjean <s.grosjean@peak-system.com>]

Hi Oliver,

(sending this message again, first was in HTML... Sorry for the noise)

Thanks for your quick answer.
Meanwhile, I finally found by myself how to get the "hardware" 
timestamps the (peak_usb) driver is saving for each CAN frame it receives...
Comparing to your candump patch, there are several things a bit 
different I want to discuss about:

Your patch enables to get the (so-called) "hardware raw timestamp" as 
well as the "hardware system transformed timestamp", right ?

When I look into net/socket.c, both timestamps are copied by 
"__sock_recv_timestamp()", according to:

- SOCK_TIMESTAMPING_RAW_HARDWARE flag ("shhwtstamps->hwtstamp" is copied 
into the skb ctrlmsg)
- SOCK_TIMESTAMPING_SYS_HARDWARE flag ("shhwtstamps->syststamp" is 
copied into the skb ctrlmsg)

If I understand well:

"hwtstamp" is the RAW timestamp received from the CAN device, without 
ANY transformation, while "syststamp" is a "system transformed hardware 
timestamp", which I understand as a timestamp the CAN device driver has 
computed (according to the "raw timestamp" for example), to be 
consistent with the system/kernel time.

So, a well-mannered linux-can device driver should (in case it is able 
to get hardware timestamps from its device):

- store in "hwstamp" the raw value received from the device
- store in "syststamp" the "transformed" value the driver computes

Is it right?

Regards,

Stéphane



Le 06/03/2014 10:56, Oliver Hartkopp a écrit :
> Hi Stephane,
>
> here's a discussion thread from last year focussing HW timestamping:
>
> http://marc.info/?l=linux-can&m=138210737431112&w=2
>
> Additionally I attached two patches:
>
> cants-test.patch:
> added some functionality to add a monotonic (sw) timestamp to sja1000
>
> candump-hwts.patch:
> added some functionality to candump to get these timestamps
>
> The candump patch is from the time before the include file cleanup inside the
> can-utils provided by Uwe. (including from socketcan directory)
>
> Together with the discussion thread from above, this should give a good
> starting point.
>
> Regards,
> Oliver
>
>
> On 06.03.2014 10:25, Stephane Grosjean wrote:
>> Hi linux-can team,
>>
>> I've got one question about how timestamping is done now in linux-can,
>> regarding to HW timestamps.
>>
>> In the early ages (~v3.4), the CAN hardware driver could set the timestamp of
>> an skb by itself and push it with the received CAN frame, so that the
>> application could get it using SO_TIMESTAMP socket option, right?
>> Now, this "hardware" timestamp is to be copied into "hwstamp" field of the
>> "skb_hwtstamps(skb)" area.
>>
>> But how does user application manage to get this hardware timestamp on its
>> side? AFAIK, the "candump" can-utils utility always reads and displays the
>> "network" timestamp (that is, always uses SO_TIMESTAMP socket option). I had a
>> quick look to the Kernel sources and tried to find the links between things
>> but it's not very clear to me: first idea I tested was to set the
>> SO_TIMESTAMPING socket option, but candump never received any hw timestamp in
>> the control messages he reads from the CAN socket...
>>
>> Any help would be appreciated.
>>
>> Regards,
>>
>> Stéphane
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PEAK-System Technik GmbH, Otto-Roehm-Strasse 69, D-64293 Darmstadt 
Geschaeftsleitung: A.Gach/U.Wilhelm,St.Nr.:007/241/13586 FA Darmstadt 
HRB-9183 Darmstadt, Ust.IdNr.:DE 202220078, WEE-Reg.-Nr.: DE39305391 
Tel.+49 (0)6151-817320 / Fax:+49 (0)6151-817329, info@peak-system.com