linux-kernel - Re: [PATCH v4] pps: Add elapsed realtime timestamping

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Date:   Mon, 20 Mar 2023 09:59:38 +0100
From:   Rodolfo Giometti <giometti@...eenne.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Alex Komrakov <alexander.komrakov@...adcom.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] pps: Add elapsed realtime timestamping

On 17/03/23 18:06, Greg KH wrote:
> On Fri, Mar 17, 2023 at 05:34:28PM +0100, Rodolfo Giometti wrote:
>> On 17/03/23 15:22, Greg KH wrote:
>>> On Fri, Mar 17, 2023 at 03:04:31PM +0100, Rodolfo Giometti wrote:
>>>> On 17/03/23 10:51, Alex Komrakov wrote:
>>>>>> +     if (!(pps->info.mode & PPS_CAPTURECLEAR))
>>>>>> +             return 0;   Why are you not returning an error?
>>>>> [AK] I used the style in this file sysfs.c.
>>>>>     assert_show() and clear_show()  have the same condition.
>>>>> When '& PPS_CAPTURECLEAR' -- 0 means no interrupt asserted  and it is not error
>>>>> Probably Rodolfo can get more info why return 0
>>>>
>>>> It's just as Alex said, if the PPS source has no PPS_CAPTUREASSERT or
>>>> PPS_CAPTURECLEAR mode it should not print ASSERT and CLEAR info.
>>>
>>> But shouldn't you return an error instead of an empty string?
>>
>> This is not an error... it's just a disabled capability. :)
> 
> Then maybe return "0" or something like that?

Yes, it could be a valid solution.

>>>>> And why are these sysfs files even present if the mode is not set
>>>>> properly?  Can the mode be set while the device is attached or is this
>>>>> only defined at probe time?  If at probe time, just never create these
>>>>> files.
>>>>> [AK] we can understand mode is set when interrupts asserted and
>>>>> file assert_elapsed will be updated.
>>>>
>>>> PPS source's "mode bits" can be set at runtime via PPS_SETPARAMS.
>>>
>>> Ok, that's good to know.  But I think the error return value is a better
>>> indication that something went wrong here and this attribute does not
>>> work for this device at this point in time.
>>
>> I see... however I suppose several code relays on this behavior.
> 
> If that's the case, then you are right, you can't change it, and I'll
> stop complaining here :)
> 
> What tools use these sysfs files?

Mainly are debugging tools via scripts. Normal usage should be via C API.

> How did you test your changes?

As above, I use scripts whose get access to sysfs to test a PPS source 
functionality. Regarding the C API I use the pps-tools:

https://github.com/redlab-i/pps-tools

Ciao,

Rodolfo

-- 
GNU/Linux Solutions                  e-mail: giometti@...eenne.com
Linux Device Driver                          giometti@...ux.it
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UNIX programming                     skype:  rodolfo.giometti