Subject: Re: [PATCH] ptp_qoriq: fix latency in ptp_qoriq_adjtime() operation.

From: Jakub Kicinski <[email-masked]>
Date: Wed, 11 Jan 2023 21:08:56 -0800

please put [PATCH net-next] in the subject.

On Tue, 10 Jan 2023 17:00:24 +0530 [email-masked] wrote:
> From: Nikhil Gupta <[email-masked]>
> 
> 1588 driver loses about 1us in adjtime operation at PTP slave.
> This is because adjtime operation uses a slow non-atomic tmr_cnt_read()
> followed by tmr_cnt_write() operation.

So far so good..

> In the above sequence, since the timer counter operation loses about 1us.

s/operation/keeps incrementing after the read/ ?

but frankly I don't think this sentence adds much

> Instead, tmr_offset register should be programmed with the delta
> nanoseconds 

missing full stop at the end.
You should describe what the tmr_offset register does.

> This won't lead to timer counter stopping and losing time
> while tmr_cnt_write() is being done.

Stopping? The timer was actually stopping?

> This Patch adds api for tmr_offset_read/write to program the

Use imperative mood.

> delta nanoseconds in the Timer offset Register.
> 
> Signed-off-by: Nikhil Gupta <[email-masked]>
> Reviewed-by: Yangbo Lu <[email-masked]>
> ---
>  drivers/ptp/ptp_qoriq.c | 36 ++++++++++++++++++++++++++++++------
>  1 file changed, 30 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/ptp/ptp_qoriq.c b/drivers/ptp/ptp_qoriq.c
> index 08f4cf0ad9e3..5b6ea6d590be 100644
> --- a/drivers/ptp/ptp_qoriq.c
> +++ b/drivers/ptp/ptp_qoriq.c
> @@ -48,6 +48,29 @@ static void tmr_cnt_write(struct ptp_qoriq *ptp_qoriq, u64 ns)
>  	ptp_qoriq->write(&regs->ctrl_regs->tmr_cnt_h, hi);
>  }
>  
> +static void tmr_offset_write(struct ptp_qoriq *ptp_qoriq, u64 delta_ns)
> +{
> +	struct ptp_qoriq_registers *regs = &ptp_qoriq->regs;
> +	u32 hi = delta_ns >> 32;
> +	u32 lo = delta_ns & 0xffffffff;
> +
> +	ptp_qoriq->write(&regs->ctrl_regs->tmroff_l, lo);
> +	ptp_qoriq->write(&regs->ctrl_regs->tmroff_h, hi);
> +}
> +
> +static u64 tmr_offset_read(struct ptp_qoriq *ptp_qoriq)
> +{
> +	struct ptp_qoriq_registers *regs = &ptp_qoriq->regs;
> +	u64 ns;
> +	u32 lo, hi;

Order variable lines longest to shortest 

> +	lo = ptp_qoriq->read(&regs->ctrl_regs->tmroff_l);
> +	hi = ptp_qoriq->read(&regs->ctrl_regs->tmroff_h);
> +	ns = ((u64) hi) << 32;
> +	ns |= lo;
> +	return ns;
> +}
> +
>  /* Caller must hold ptp_qoriq->lock. */
>  static void set_alarm(struct ptp_qoriq *ptp_qoriq)
>  {
> @@ -55,7 +78,9 @@ static void set_alarm(struct ptp_qoriq *ptp_qoriq)
>  	u64 ns;
>  	u32 lo, hi;
>  
> -	ns = tmr_cnt_read(ptp_qoriq) + 1500000000ULL;
> +	ns = tmr_cnt_read(ptp_qoriq) + tmr_offset_read(ptp_qoriq)
> +				     + 1500000000ULL;
> +
>  	ns = div_u64(ns, 1000000000UL) * 1000000000ULL;
>  	ns -= ptp_qoriq->tclk_period;
>  	hi = ns >> 32;
> @@ -207,15 +232,12 @@ EXPORT_SYMBOL_GPL(ptp_qoriq_adjfine);
>  
>  int ptp_qoriq_adjtime(struct ptp_clock_info *ptp, s64 delta)
>  {
> -	s64 now;
>  	unsigned long flags;
>  	struct ptp_qoriq *ptp_qoriq = container_of(ptp, struct ptp_qoriq, caps);
>  
>  	spin_lock_irqsave(&ptp_qoriq->lock, flags);
>  
> -	now = tmr_cnt_read(ptp_qoriq);
> -	now += delta;
> -	tmr_cnt_write(ptp_qoriq, now);
> +	tmr_offset_write(ptp_qoriq, delta);

Writes to the offset register result in an add operation? 
Or it's a pure write? What will the offset be after a sequence of
following adjtime() calls: 
  adjtime(+100); 
  adjtime(+100); 
  adjtime(+100);
?

>  	set_fipers(ptp_qoriq);
>  
>  	spin_unlock_irqrestore(&ptp_qoriq->lock, flags);
> @@ -232,7 +254,7 @@ int ptp_qoriq_gettime(struct ptp_clock_info *ptp, struct timespec64 *ts)
>  
>  	spin_lock_irqsave(&ptp_qoriq->lock, flags);
>  
> -	ns = tmr_cnt_read(ptp_qoriq);
> +	ns = tmr_cnt_read(ptp_qoriq) + tmr_offset_read(ptp_qoriq);
>  
>  	spin_unlock_irqrestore(&ptp_qoriq->lock, flags);
>  
> @@ -251,6 +273,8 @@ int ptp_qoriq_settime(struct ptp_clock_info *ptp,
>  
>  	ns = timespec64_to_ns(ts);
>  
> +	tmr_offset_write(ptp_qoriq, 0);

Shouldn't this be under the lock?

>  	spin_lock_irqsave(&ptp_qoriq->lock, flags);
>  
>  	tmr_cnt_write(ptp_qoriq, ns);