Re: [PATCH v2 2/6] bus: mhi: host: Add support for non-posted TSC timesync feature

[Vadim Fedorenko <vadim.fedorenko@linux.dev>]

On 11/04/2026 09:12, Krishna Chaitanya Chundru wrote:
> From: Vivek Pernamitta <quic_vpernami@quicinc.com>
> 
> Implement non-posted time synchronization as described in section 5.1.1
> of the MHI v1.2 specification. The host disables low-power link states
> to minimize latency, reads the local time, issues a MMIO read to the
> device's TIME register.
> 
> Add support for initializing this feature and export a function to be
> used by the drivers which does the time synchronization.
> 
> MHI reads the device time registers in the MMIO address space pointed to
> by the capability register after disabling all low power modes and keeping
> MHI in M0. Before and after MHI reads, the local time is captured
> and shared for processing.

[...]

> +	/*
> +	 * time critical code to fetch device time, delay between these two steps
> +	 * should be deterministic as possible.
> +	 */
> +	preempt_disable();
> +	local_irq_disable();
> +
> +	time->t_host_pre = ktime_get_real();
> +
> +	/*
> +	 * To ensure the PCIe link is in L0 when ASPM is enabled, perform series
> +	 * of back-to-back reads. This is necessary because the link may be in a
> +	 * low-power state (e.g., L1 or L1ss), and need to be forced it to
> +	 * transition to L0.
> +	 */
> +	for (i = 0; i < MHI_NUM_BACK_TO_BACK_READS; i++) {
> +		ret = mhi_read_reg(mhi_cntrl, mhi_tsync->time_reg,
> +				   TSC_TIMESYNC_TIME_LOW_OFFSET, &time->t_dev_lo);
> +
> +		ret = mhi_read_reg(mhi_cntrl, mhi_tsync->time_reg,
> +				   TSC_TIMESYNC_TIME_HIGH_OFFSET, &time->t_dev_hi);
> +	}
> +
> +	time->t_host_post = ktime_get_real();
> +
> +	local_irq_enable();
> +	preempt_enable();

PTP_SYS_OFFSET_EXTENDED receives the amount of samples to read from user
space, you can use it instead of MHI_NUM_BACK_TO_BACK_READS, and in this
case it's better to grab host-pre and host-post time for a single
register read.

Also, PTP_SYS_OFFSET_EXTENDED was improved and currently supports
multiple clockids as system time, it's good to account for it.