Re: [PATCH iwl-net] igc: Ensure PTM request is completed before timeout has started

[Kurt Kanzenbach <kurt.kanzenbach@linutronix.de>]

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On Mon Jul 08 2024, Rodrigo Cataldo via B4 Relay wrote:
> From: Rodrigo Cataldo <rodrigo.cadore@l-acoustics.com>
>
> When a PTM is requested via wr32(IGC_PTM_STAT), the operation may only
> be completed by the next read operation (flush). Unfortunately, the next
> read operation in the PTM request loop happens after we have already
> started evaluating the response timeout.
>
> Thus, the following behavior has been observed::
>
>   phc2sys-1655  [010]   103.233752: funcgraph_entry:                    |  igc_ptp_getcrosststamp() {
>   phc2sys-1655  [010]   103.233754: funcgraph_entry:                    |    igc_phc_get_syncdevice_time() {
>   phc2sys-1655  [010]   103.233755: funcgraph_entry:                    |      igc_rd32() {
>   phc2sys-1655  [010]   103.233931: preempt_disable: caller=irq_enter_rcu+0x14 parent=irq_enter_rcu+0x14
>   phc2sys-1655  [010]   103.233932: local_timer_entry: vector=236
>   phc2sys-1655  [010]   103.233932: hrtimer_cancel: hrtimer=0xffff8edeef526118
>   phc2sys-1655  [010]   103.233933: hrtimer_expire_entry: hrtimer=0xffff8edeef526118 now=103200127876 function=tick_nohz_handler/0x0
>
>   ... tick handler ...
>
>   phc2sys-1655  [010]   103.233971: funcgraph_exit:       !  215.559 us |      }
>   phc2sys-1655  [010]   103.233972: funcgraph_entry:                    |      igc_rd32() {
>   phc2sys-1655  [010]   103.234135: funcgraph_exit:       !  164.370 us |      }
>   phc2sys-1655  [010]   103.234136: funcgraph_entry:         1.942 us   |      igc_rd32();
>   phc2sys-1655  [010]   103.234147: console:              igc 0000:03:00.0 enp3s0: Timeout reading IGC_PTM_STAT register
>
> Based on the (simplified) code::
>
> 	ctrl = rd32(IGC_PTM_CTRL);
>         /* simplified: multiple writes here */
> 	wr32(IGC_PTM_STAT, IGC_PTM_STAT_VALID);
>
> 	err = readx_poll_timeout(rd32, IGC_PTM_STAT, stat,
> 				 stat, IGC_PTM_STAT_SLEEP,
> 				 IGC_PTM_STAT_TIMEOUT);
> 	if (err < 0) {
> 		netdev_err(adapter->netdev, "Timeout reading IGC_PTM_STAT register\n");
> 		return err;
> 	}
>
> Where readx_poll_timeout() starts the timeout evaluation before calling
> the rd32() parameter (rd32() is a macro for igc_rd32()).
>
> In the trace shown, the read operation of readx_poll_timeout() (second
> igc_rd32()) took so long that the timeout (IGC_PTM_STAT_VALID) has expired
> and no sleep has been performed.
>
> With this patch, a write flush is added (which is an additional
> igc_rd32() in practice) that can wait for the write before the timeout
> is evaluated::
>
>   phc2sys-1615  [010]    74.517954: funcgraph_entry:                    |  igc_ptp_getcrosststamp() {
>   phc2sys-1615  [010]    74.517956: funcgraph_entry:                    |    igc_phc_get_syncdevicetime() {
>   phc2sys-1615  [010]    74.517957: funcgraph_entry:                    |      igc_rd32() {
>   phc2sys-1615  [010]    74.518127: preempt_disable: caller=irq_enter_rcu+0x14 parent=irq_enter_rcu+0x14
>   phc2sys-1615  [010]    74.518128: local_timer_entry: vector=236
>   phc2sys-1615  [010]    74.518128: hrtimer_cancel: hrtimer=0xffff96466f526118
>   phc2sys-1615  [010]    74.518128: hrtimer_expire_entry: hrtimer=0xffff96466f526118 now=74484007229 function=tick_nohz_handler/0x0
>
>   ... tick handler ...
>
>   phc2sys-1615  [010]    74.518180: funcgraph_exit:       !  222.282 us |      }
>   phc2sys-1615  [010]    74.518181: funcgraph_entry:                    |      igc_rd32() {
>   phc2sys-1615  [010]    74.518349: funcgraph_exit:       !  168.160 us |      }
>   phc2sys-1615  [010]    74.518349: funcgraph_entry:         1.970 us   |      igc_rd32();
>   phc2sys-1615  [010]    74.518352: hrtimer_init: hrtimer=0xffffa6f9413a3940 clockid=CLOCK_MONOTONIC mode=0x0
>   phc2sys-1615  [010]    74.518352: preempt_disable: caller=_raw_spin_lock_irqsave+0x28 parent=hrtimer_start_range_ns+0x56
>   phc2sys-1615  [010]    74.518353: hrtimer_start: hrtimer=0xffffa6f9413a3940 function=hrtimer_wakeup/0x0 expires=74484232878 softexpires=74484231878
>
>   .. hrtimer setup and return ...
>
>   kworker/10:1-242   [010]    74.518382: sched_switch: kworker/10:1:242 [120] W ==> phc2sys:1615 [120]
>   phc2sys-1615  [010]    74.518383: preempt_enable: caller=schedule+0x36 parent=schedule+0x36
>   phc2sys-1615  [010]    74.518384: funcgraph_entry:      !  100.088 us |      igc_rd32();
>   phc2sys-1615  [010]    74.518484: funcgraph_entry:         1.958 us   |      igc_rd32();
>   phc2sys-1615  [010]    74.518488: funcgraph_entry:         2.019 us   |      igc_rd32();
>   phc2sys-1615  [010]    74.518490: funcgraph_entry:         1.956 us   |      igc_rd32();
>   phc2sys-1615  [010]    74.518492: funcgraph_entry:         1.980 us   |      igc_rd32();
>   phc2sys-1615  [010]    74.518494: funcgraph_exit:       !  539.386 us |    }
>
> Now the sleep is called as expected, and the operation succeeds.
> Therefore, regardless of how long it will take for the write to be
> completed, we will poll+sleep at least for the time specified in
> IGC_PTM_STAT_TIMEOUT.
>
> Fixes: a90ec8483732 ("igc: Add support for PTP getcrosststamp()")
> Signed-off-by: Rodrigo Cataldo <rodrigo.cadore@l-acoustics.com>

Thanks for sending this upstream.

Reviewed-by: Kurt Kanzenbach <kurt@linutronix.de>

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