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

On Sat, Apr 11, 2026 at 01:42:02PM +0530, Krishna Chaitanya Chundru wrote:
> From: Vivek Pernamitta <[email protected]>
> 
> 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.
> 
> Signed-off-by: Vivek Pernamitta <[email protected]>
> Signed-off-by: Krishna Chaitanya Chundru <[email protected]>
> ---
>  drivers/bus/mhi/common.h        |  4 +++
>  drivers/bus/mhi/host/init.c     | 28 ++++++++++++++++
>  drivers/bus/mhi/host/internal.h |  9 +++++
>  drivers/bus/mhi/host/main.c     | 74 +++++++++++++++++++++++++++++++++++++++++
>  include/linux/mhi.h             | 37 +++++++++++++++++++++
>  5 files changed, 152 insertions(+)
> 
> diff --git a/drivers/bus/mhi/common.h b/drivers/bus/mhi/common.h
> index 4c316f3d5a68beb01f15cf575b03747096fdcf2c..64f9b2b94387a112bb6b5e20c634c3ba8d6bc78e 100644
> --- a/drivers/bus/mhi/common.h
> +++ b/drivers/bus/mhi/common.h
> @@ -118,6 +118,10 @@
>  #define CAP_CAPID_MASK			GENMASK(31, 24)
>  #define CAP_NEXT_CAP_MASK		GENMASK(23, 12)
>  
> +/* MHI TSC Timesync */
> +#define TSC_TIMESYNC_TIME_LOW_OFFSET	(0x8)
> +#define TSC_TIMESYNC_TIME_HIGH_OFFSET	(0xC)
> +
>  /* Command Ring Element macros */
>  /* No operation command */
>  #define MHI_TRE_CMD_NOOP_PTR		0
> diff --git a/drivers/bus/mhi/host/init.c b/drivers/bus/mhi/host/init.c
> index c2162aa04e810e45ccfbedd20aaa62f892420d31..eb720f671726d919646cbc450cd54bda655a1060 100644
> --- a/drivers/bus/mhi/host/init.c
> +++ b/drivers/bus/mhi/host/init.c
> @@ -498,6 +498,30 @@ static int mhi_find_capability(struct mhi_controller *mhi_cntrl, u32 capability)
>  	return 0;
>  }
>  
> +static int mhi_init_tsc_timesync(struct mhi_controller *mhi_cntrl)
> +{
> +	struct device *dev = &mhi_cntrl->mhi_dev->dev;
> +	struct mhi_timesync *mhi_tsc_tsync;
> +	u32 time_offset;
> +	int ret;
> +
> +	time_offset = mhi_find_capability(mhi_cntrl, MHI_CAP_ID_TSC_TIME_SYNC);
> +	if (!time_offset)
> +		return -ENXIO;
> +
> +	mhi_tsc_tsync = devm_kzalloc(dev, sizeof(*mhi_tsc_tsync), GFP_KERNEL);
> +	if (!mhi_tsc_tsync)
> +		return -ENOMEM;
> +
> +	mhi_cntrl->tsc_timesync = mhi_tsc_tsync;
> +	mutex_init(&mhi_tsc_tsync->ts_mutex);
> +
> +	/* save time_offset for obtaining time via MMIO register reads */
> +	mhi_tsc_tsync->time_reg = mhi_cntrl->regs + time_offset;
> +
> +	return 0;
> +}
> +
>  int mhi_init_mmio(struct mhi_controller *mhi_cntrl)
>  {
>  	u32 val;
> @@ -635,6 +659,10 @@ int mhi_init_mmio(struct mhi_controller *mhi_cntrl)
>  		return ret;
>  	}
>  
> +	ret = mhi_init_tsc_timesync(mhi_cntrl);
> +	if (ret)
> +		dev_dbg(dev, "TSC Time synchronization init failure\n");
> +
>  	return 0;
>  }
>  
> diff --git a/drivers/bus/mhi/host/internal.h b/drivers/bus/mhi/host/internal.h
> index 7b0ee5e3a12dd585064169b7b884750bf4d8c8db..a0e729e7a1198c1b82c70b6bfe3bc2ee24331229 100644
> --- a/drivers/bus/mhi/host/internal.h
> +++ b/drivers/bus/mhi/host/internal.h
> @@ -15,6 +15,15 @@ extern const struct bus_type mhi_bus_type;
>  #define MHI_SOC_RESET_REQ_OFFSET			0xb0
>  #define MHI_SOC_RESET_REQ				BIT(0)
>  
> +/*
> + * With ASPM enabled, the link may enter a low power state, requiring
> + * a wake-up sequence. Use a short burst of back-to-back reads to
> + * transition the link to the active state. Based on testing,
> + * 4 iterations are necessary to ensure reliable wake-up without
> + * excess latency.
> + */
> +#define MHI_NUM_BACK_TO_BACK_READS			4
> +
>  struct mhi_ctxt {
>  	struct mhi_event_ctxt *er_ctxt;
>  	struct mhi_chan_ctxt *chan_ctxt;
> diff --git a/drivers/bus/mhi/host/main.c b/drivers/bus/mhi/host/main.c
> index 53c0ffe300702bcc3caa8fd9ea8086203c75b186..b7a727b1a5d1f20b570c62707a991ec5b85bfec7 100644
> --- a/drivers/bus/mhi/host/main.c
> +++ b/drivers/bus/mhi/host/main.c
> @@ -1626,3 +1626,77 @@ int mhi_get_channel_doorbell_offset(struct mhi_controller *mhi_cntrl, u32 *chdb_
>  	return 0;
>  }
>  EXPORT_SYMBOL_GPL(mhi_get_channel_doorbell_offset);
> +
> +static int mhi_get_remote_time(struct mhi_controller *mhi_cntrl, struct mhi_timesync *mhi_tsync,
> +			       struct mhi_timesync_info *time)
> +{
> +	struct device *dev = &mhi_cntrl->mhi_dev->dev;
> +	int ret, i;
> +
> +	if (!mhi_tsync && !mhi_tsync->time_reg) {
> +		dev_err(dev, "Time sync is not supported\n");
> +		return -EINVAL;
> +	}
> +
> +	if (unlikely(MHI_PM_IN_ERROR_STATE(mhi_cntrl->pm_state))) {
> +		dev_err(dev, "MHI is not in active state, pm_state:%s\n",
> +			to_mhi_pm_state_str(mhi_cntrl->pm_state));
> +		return -EIO;
> +	}
> +
> +	/* bring to M0 state */
> +	ret = mhi_device_get_sync(mhi_cntrl->mhi_dev);
> +	if (ret)
> +		return ret;
> +
> +	guard(mutex)(&mhi_tsync->ts_mutex);
> +	mhi_cntrl->runtime_get(mhi_cntrl);
> +
> +	/*
> +	 * time critical code to fetch device time, delay between these two steps
> +	 * should be deterministic as possible.
> +	 */

Thinking more, to be deterministic, ASPM should be disabled at this point. But
we cannot do that safely now from PCI client drivers until this series gets in:
https://lore.kernel.org/linux-arm-msm/[email protected]/

So let's keep it this way for now, but add a FIXME to disable ASPM.

> +	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);

And an idiomatic way to read 64bit MMIO value in two 32bit read is:

	do {
		high = read_high();
		low = read_low();
	} while(high != read_high());

- Mani

-- 
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