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

Vadim Fedorenko
2026-04-14T17:46:22Z

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.