netdev - RE: [Intel-wired-lan] [PATCH v10 iwl-next 05/11] idpf: negotiate PTP capabilities and get PTP clock

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Message-ID: <MW4PR11MB588927730666406EEDA9B5A58EB42@MW4PR11MB5889.namprd11.prod.outlook.com>
Date: Wed, 9 Apr 2025 12:55:58 +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>, Willem de Bruijn <willemb@...gle.com>, "Mina
 Almasry" <almasrymina@...gle.com>, "Salin, Samuel" <samuel.salin@...el.com>
Subject: RE: [Intel-wired-lan] [PATCH v10 iwl-next 05/11] idpf: negotiate PTP
 capabilities and get PTP clock

On 4/8/2025 11:15 PM, Jacob Keller wrote:

>On 4/8/2025 3:30 AM, Milena Olech wrote:
>> +static u64 idpf_ptp_read_src_clk_reg_direct(struct idpf_adapter *adapter,
>> +					    struct ptp_system_timestamp *sts)
>> +{
>> +	struct idpf_ptp *ptp = adapter->ptp;
>> +	u32 hi, lo;
>> +
>> +	spin_lock(&ptp->read_dev_clk_lock);
>> +
>> +	/* Read the system timestamp pre PHC read */
>> +	ptp_read_system_prets(sts);
>> +
>> +	idpf_ptp_enable_shtime(adapter);
>> +
>> +	/* Read the system timestamp post PHC read */
>> +	ptp_read_system_postts(sts);
>> +
>> +	lo = readl(ptp->dev_clk_regs.dev_clk_ns_l);
>> +	hi = readl(ptp->dev_clk_regs.dev_clk_ns_h);
>> +
>> +	spin_unlock(&ptp->read_dev_clk_lock);
>> +
>> +	return ((u64)hi << 32) | lo;
>> +}
>v9 had comments regarding the latching of the registers for direct
>access. Can you confirm whether this is known to be safe, or if you need
>to implement a 3-part read like we do in ice and other hardware? Even
>with a spinlock there could be issues with rollover in the hardware I think?
>

So in this model we have shadow registers and we trigger HW - by writes
executed in idpf_ptp_enable_shtime - to latch the value. I've made some
experiments, where I removed this function call, and values in hi/lo
registers are constantly the same.

In other words, it is safe to read values from hi/lo registers until
the next latch.

To my best knowledge, ice does not have any HW support, that's why all
these actions are required.

Milena

>Thanks,
>Jake
>