netdev - Re: [PATCH] ptp: ocp: Fix PCI delay estimation

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Message-ID: <20250818213104.34351-1-antoine@gagniere.dev>
Date: Mon, 18 Aug 2025 23:31:04 +0200
From: Antoine Gagniere <antoine@...niere.dev>
To: vadim.fedorenko@...ux.dev
Cc: antoine@...niere.dev,
	jonathan.lemon@...il.com,
	kuba@...nel.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH] ptp: ocp: Fix PCI delay estimation

>>   static void
>>   ptp_ocp_estimate_pci_timing(struct ptp_ocp *bp)
>>   {
>> -	ktime_t start, end, delay = U64_MAX;
>> +	ktime_t start, end;
>> +	s64 delay_ns = U32_MAX; /* 4.29 seconds is high enough */
>>   	u32 ctrl;
>>   	int i;
>>
>> @@ -1568,15 +1569,16 @@ ptp_ocp_estimate_pci_timing(struct ptp_ocp *bp)
>>
>>   		iowrite32(ctrl, &bp->reg->ctrl);
>>
>> -		start = ktime_get_raw_ns();
>> +		start = ktime_get_raw();
>>
>>   		ctrl = ioread32(&bp->reg->ctrl);
>>
>> -		end = ktime_get_raw_ns();
>> +		end = ktime_get_raw();
>>
>> -		delay = min(delay, end - start);
>> +		delay_ns = min(delay_ns, ktime_to_ns(end - start));
>>   	}
>> -	bp->ts_window_adjust = (delay >> 5) * 3;
>> +	delay_ns = max(0, delay_ns);0
>
> I don't believe we can get a negative value from
> ktime_to_ns(end - start), and that means that delay_ns will always be
> positive and there is no need for the last max().

You are correct, ktime is monotonic, this was an excess of zeal
on my part.


> JFYI, for the next version could you please organize the commit message
> to fit into 80 chars per line and specify net tree as well as patch
> which you believe introduced the problem using Fixes tag.
>
> More details on formatting can be found at
> https://docs.kernel.org/process/submitting-patches.html#submittingpatches

Thanks for the link, I was looking for such documentation but somehow missed this page

Will attempt a v2 later this week