netdev - RE: [Intel-wired-lan] [PATCH v10 iwl-next 06/11] idpf: add mailbox access to read PTP clock time

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Message-ID: <MW4PR11MB58896B0420FE0EDCBBAA67D08EB42@MW4PR11MB5889.namprd11.prod.outlook.com>
Date: Wed, 9 Apr 2025 13:01:21 +0000
From: "Olech, Milena" <milena.olech@...el.com>
To: "Keller, Jacob E" <jacob.e.keller@...el.com>,
	"intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>, "Nguyen, Anthony L"
	<anthony.l.nguyen@...el.com>, "Kitszel, Przemyslaw"
	<przemyslaw.kitszel@...el.com>, "Lobakin, Aleksander"
	<aleksander.lobakin@...el.com>, Willem de Bruijn <willemb@...gle.com>,
	"Salin, Samuel" <samuel.salin@...el.com>
Subject: RE: [Intel-wired-lan] [PATCH v10 iwl-next 06/11] idpf: add mailbox
 access to read PTP clock time

On 4/8/2025 11:17 PM, Jacob Keller wrote:

>On 4/8/2025 3:30 AM, Milena Olech wrote:
>> +static int idpf_ptp_read_src_clk_reg_mailbox(struct idpf_adapter *adapter,
>> +					     struct ptp_system_timestamp *sts,
>> +					     u64 *src_clk)
>> +{
>> +	struct idpf_ptp_dev_timers clk_time;
>> +	int err;
>> +
>> +	/* Read the system timestamp pre PHC read */
>> +	ptp_read_system_prets(sts);
>> +
>> +	err = idpf_ptp_get_dev_clk_time(adapter, &clk_time);
>> +	if (err)
>> +		return err;
>> +
>> +	/* Read the system timestamp post PHC read */
>> +	ptp_read_system_postts(sts);
>> +
>> +	*src_clk = clk_time.dev_clk_time_ns;
>> +
>> +	return 0;
>> +}
>Not an objection, since its obvious using the direct register access is
>preferable when it is available. This will result in a fair amount of
>jitter since mailbox timing is unlikely to be consistent. We also cannot
>have sts be filled in somehow by the host because it might not be
>operating on the same system clock.
>

You're 100% right, reading the time of the clock through mailbox is not
efficient. But as you mentioned, direct register access is definitely
preferable.

Thanks,
Milena