Subject: Re: [PATCH net-next v4 3/3] gve: implement PTP gettimex64

From: Jakub Kicinski <[email-masked]>
Date: Thu, 9 Apr 2026 20:16:46 -0700

On Mon,  6 Apr 2026 23:40:02 +0000 Harshitha Ramamurthy wrote:
> From: Jordan Rhee <[email-masked]>
> 
> Enable chrony and phc2sys to synchronize system clock to NIC clock.
> 
> The system cycle counters are sampled by the device to minimize the
> uncertainty window. If the system times are sampled in the host, the
> delta between pre and post readings is 100us or more due to AQ command
> latency. The system times returned by the device have a delta of ~1us,
> which enables significantly more accurate clock synchronization.

Interesting. I'd like this looked over by David Woodhouse and tglx.
Please repost after the merge window or send them an RFC.

> +static int gve_ptp_read_timestamp(struct gve_ptp *ptp, cycles_t *pre_cycles,
> +				  cycles_t *post_cycles,
> +				  struct system_time_snapshot *snap)
> +{
> +	unsigned long delay_us = 1000;
> +	int retry_count = 0;
> +	int err;
> +
> +	lockdep_assert_held(&ptp->nic_ts_read_lock);
> +
> +	do {

This can't be a for () loop with 5 iterations?

> +		if (snap)
> +			ktime_get_snapshot(snap);
> +
> +		*pre_cycles = get_cycles();
> +		err = gve_adminq_report_nic_ts(ptp->priv,
> +					       ptp->nic_ts_report_bus);
> +
> +		/* Prevent get_cycles() from being speculatively executed
> +		 * before the AdminQ command
> +		 */
> +		rmb();
> +		*post_cycles = get_cycles();
> +		if (likely(err != -EAGAIN))
> +			return err;
> +
> +		fsleep(delay_us);
> +
> +		/* Exponential backoff */
> +		delay_us *= 2;
> +		retry_count++;
> +	} while (retry_count < 5);
> +
> +	return -ETIMEDOUT;
> +}
> +
>  /* Read the nic timestamp from hardware via the admin queue. */
> -static int gve_clock_nic_ts_read(struct gve_ptp *ptp, u64 *nic_raw)
> +static int gve_clock_nic_ts_read(struct gve_ptp *ptp, u64 *nic_raw,
> +				 struct gve_sysclock_sample *sysclock)
>  {
> +	cycles_t host_pre_cycles, host_post_cycles;
> +	struct gve_nic_ts_report *ts_report;
>  	int err;
>  
>  	mutex_lock(&ptp->nic_ts_read_lock);
> -	err = gve_adminq_report_nic_ts(ptp->priv, ptp->nic_ts_report_bus);
> -	if (err)
> +	err = gve_ptp_read_timestamp(ptp, &host_pre_cycles, &host_post_cycles,
> +				     sysclock ? &sysclock->snapshot : NULL);
> +	if (err) {
> +		dev_err_ratelimited(&ptp->priv->pdev->dev,
> +				    "AdminQ timestamp read failed: %d\n", err);
>  		goto out;
> +	}
>  
> -	*nic_raw = be64_to_cpu(ptp->nic_ts_report->nic_timestamp);
> +	ts_report = ptp->nic_ts_report;
> +	*nic_raw = be64_to_cpu(ts_report->nic_timestamp);
> +
> +	if (sysclock) {
> +		sysclock->nic_pre_cycles = be64_to_cpu(ts_report->pre_cycles);
> +		sysclock->nic_post_cycles = be64_to_cpu(ts_report->post_cycles);
> +		sysclock->host_pre_cycles = host_pre_cycles;
> +		sysclock->host_post_cycles = host_post_cycles;
> +	}
>  
>  out:
>  	mutex_unlock(&ptp->nic_ts_read_lock);
>  	return err;
>  }
>  
> +struct gve_cycles_to_clock_callback_ctx {
> +	u64 cycles;
> +};
> +
> +static int gve_cycles_to_clock_fn(ktime_t *device_time,
> +				  struct system_counterval_t *system_counterval,
> +				  void *ctx)

Does this do anything GVE specific??

> +{
> +	struct gve_cycles_to_clock_callback_ctx *context = ctx;
> +
> +	*device_time = 0;
> +
> +	system_counterval->cycles = context->cycles;
> +	system_counterval->use_nsecs = false;
> +
> +	if (IS_ENABLED(CONFIG_X86))
> +		system_counterval->cs_id = CSID_X86_TSC;
> +	else if (IS_ENABLED(CONFIG_ARM64))
> +		system_counterval->cs_id = CSID_ARM_ARCH_COUNTER;
> +	else
> +		return -EOPNOTSUPP;
> +
> +	return 0;
> +}
-- 
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