Re: [Intel-wired-lan] [PATCH iwl-next v3] igb: Retrieve Tx timestamp directly from interrupt for i210

[Jacob Keller]

On 2/5/2026 4:20 AM, Loktionov, Aleksandr wrote:
> > -----Original Message-----
> > From: Intel-wired-lan <intel-wired-lan-bounces@xxxxxxxxxx> On Behalf
> > Of Kurt Kanzenbach
> > Sent: Thursday, February 5, 2026 12:58 PM
> > To: Loktionov, Aleksandr <aleksandr.loktionov@xxxxxxxxx>; Nguyen,
> > Anthony L <anthony.l.nguyen@xxxxxxxxx>; Kitszel, Przemyslaw
> > <przemyslaw.kitszel@xxxxxxxxx>
> > Cc: Paul Menzel <pmenzel@xxxxxxxxxxxxx>; Vadim Fedorenko
> > <vadim.fedorenko@xxxxxxxxx>; Gomes, Vinicius
> > <vinicius.gomes@xxxxxxxxx>; netdev@xxxxxxxxxxxxxxx; Richard Cochran
> > <richardcochran@xxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; Andrew Lunn
> > <andrew+netdev@xxxxxxx>; Eric Dumazet <edumazet@xxxxxxxxxx>; intel-
> > wired-lan@xxxxxxxxxxxxxxxx; Keller, Jacob E
> > <jacob.e.keller@xxxxxxxxx>; Jakub Kicinski <kuba@xxxxxxxxxx>; Paolo
> > Abeni <pabeni@xxxxxxxxxx>; David S. Miller <davem@xxxxxxxxxxxxx>;
> > Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> > Subject: Re: [Intel-wired-lan] [PATCH iwl-next v3] igb: Retrieve Tx
> > timestamp directly from interrupt for i210
> >
> > On Thu Feb 05 2026, Loktionov, Aleksandr wrote:
> > > > +/**
> > > > + * igb_ptp_tx_tstamp_event
> > > > + * @adapter: pointer to igb adapter
> > > > + *
> > > > + * This function checks the TSYNCTXCTL valid bit and stores the Tx
> > > > +hardware
> > > > + * timestamp at the current skb.
> > > > + **/
> > > > +void igb_ptp_tx_tstamp_event(struct igb_adapter *adapter) {
> > > > +	struct e1000_hw *hw = &adapter->hw;
> > > > +	u32 tsynctxctl;
> > > > +
> > > > +	if (!adapter->ptp_tx_skb)
> > > > +		return;
> > > > +
> > > > +	tsynctxctl = rd32(E1000_TSYNCTXCTL);
> > > > +	if (WARN_ON_ONCE(!(tsynctxctl & E1000_TSYNCTXCTL_VALID)))
> > > > +		return;
> > > > +
> > > > +	igb_ptp_tx_hwtstamp(adapter); <-Calls existing function
> > designed for work queue!
> > > skb_tstamp_tx() can sleep
> > > Smells like sleep-in-atomic isn't it?
> >
> > AFAICS skb_tstamp_tx() is safe to call here.
> >
> > > spin_lock_irqsave(&wq_head->lock, flags);  <- RT mutex can sleep
> >
> > In case you're worried about PREEMPT_RT: On -RT the IRQ runs a
> > dedicated thread. BTW I've tested this with and without -RT and with
> > CONFIG_DEBUG_ATOMIC_SLEEP.
> >
> > Thanks,
> > Kurt
>
> Thank you, Kurt for sharing your experience. I don't have so many experience with RT Linux.
> For me calling a function, not designed to be called from IRQ context is a SUS.
> So, I rose the question about sleeping.

My understanding is that RT is only safe to convert such spinlock_t to 
mutex *because* it also converts IRQs to threads.