Re: [dpdk-dev] [PATCH 1/3] rte_ethdev: Add API function to read dev clock

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 3/20/2019 3:57 PM, Andrew Rybchenko wrote:
> On 3/20/19 5:48 PM, Thomas Monjalon wrote:
>> 19/03/2019 14:32, Yigit, Ferruh:
>>> On 1/8/2019 11:30 AM, Tom Barbette wrote:
>>>> Ferruh Yigit wrote:
>>>>> Why timestamp offloading become useless? When timestamp offloading enabled,
>>>>> device fills 'mbuf.timestamp' and you can use it.
>>>> But the frequency is unknown, and the reference time neither. So it can be used only to know that "some time passed" between packets.
>>>>
>>>>> For your case this timestamp for mlx is device clock and you are adding this API
>>>>> to be able to convert device clock to real time, this is not something enables
>>>>> the timestamp offload.
>>>> I get your point, but a keyboard is highly required to use a computer. It's pretty much useless without it. Without this API, the timestamp offload makes no sense. It's a random number generator at best...
>>>>
>>>>> Technically driver can set the 'mbuf.timestamp' with the real clock right, if it
>>>>> is required? Or this can be defined by a devarg?
>>>> I don't think so. Device have no sense of system time. And doing it in the driver is tricky because it depends on the user needs. Catch-up with NTP updates would need a timer and various parameters... Hence we prefer to give a simple working code, and users may do this if they want.
>>>>
>>>>
>>>> For the other comments it's not my call... I would just underline that timestamp offload is not usable in the current state, and there is a lot of use case for monitoring latency-sensitive applications.
>>> Hi Thomas, Andrew,
>>>
>>> CAn you please comment on patch, it adds a new 'rte_eth_read_clock()' API to
>>> read device clock to read timestamp value, later to use this value to map to the
>>> actual time.
>>> So that can convert timestamp information from each packet into real time.
>> The approach is smart in my opinion.
>> It is requesting the time generator (at its source) and allowing
>> the app to do any kind of time handling strategy.
>>
>>> My question was if this is common requirement or specific to single device?
>> It will work with any device providing some timestamps.
>> There is nothing specific here in my opinion.
>>
>>> And if can be handles in driver level.
>> Yes, it may be handled differently.
>> But this approach looks to be the most flexible and reliable.
>>
>> Acked-by: Thomas Monjalon <thomas@monjalon.net>
> 
> Taking into account that timestamp in mbuf is not normalized (neither
> unit nor reference) and the API helps to normalize units, it makes sense.
> I recall discussion about timestamp if should be normalized or
> not, the decision was to keep it undefined.
> 
> Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>
> 

Hi Tom,

mlx patch doesn't apply cleanly, can you please send a new version on top of
latest head?

Thanks,
ferruh