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Date: Mon, 1 Apr 2024 21:18:00 +0900
From: Takashi Sakamoto <o-takashi@sakamocchi.jp>
To: Adam Goldman <adamg@pobox.com>, linux1394-devel@lists.sourceforge.net,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] firewire: ohci: mask bus reset interrupts between ISR
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Hi Adam,

On Mon, Mar 25, 2024 at 09:58:28AM +0900, Takashi Sakamoto wrote:
> Hi,
> 
> On Wed, Mar 20, 2024 at 02:15:19AM -0700, Adam Goldman wrote:
> > In the FireWire OHCI interrupt handler, if a bus reset interrupt has 
> > occurred, mask bus reset interrupts until bus_reset_work has serviced and 
> > cleared the interrupt.
> > 
> > Normally, we always leave bus reset interrupts masked. We infer the bus 
> > reset from the self-ID interrupt that happens shortly thereafter. A 
> > scenario where we unmask bus reset interrupts was introduced in 2008 in 
> > a007bb857e0b26f5d8b73c2ff90782d9c0972620: If 
> > OHCI_PARAM_DEBUG_BUSRESETS (8) is set in the debug parameter bitmask, we 
> > will unmask bus reset interrupts so we can log them.
> > 
> > irq_handler logs the bus reset interrupt. However, we can't clear the bus 
> > reset event flag in irq_handler, because we won't service the event until 
> > later. irq_handler exits with the event flag still set. If the 
> > corresponding interrupt is still unmasked, the first bus reset will 
> > usually freeze the system due to irq_handler being called again each 
> > time it exits. This freeze can be reproduced by loading firewire_ohci 
> > with "modprobe firewire_ohci debug=-1" (to enable all debugging output). 
> > Apparently there are also some cases where bus_reset_work will get called 
> > soon enough to clear the event, and operation will continue normally.
> > 
> > This freeze was first reported a few months after a007bb85 was committed, 
> > but until now it was never fixed. The debug level could safely be set 
> > to -1 through sysfs after the module was loaded, but this would be 
> > ineffectual in logging bus reset interrupts since they were only 
> > unmasked during initialization.
> > 
> > irq_handler will now leave the event flag set but mask bus reset 
> > interrupts, so irq_handler won't be called again and there will be no 
> > freeze. If OHCI_PARAM_DEBUG_BUSRESETS is enabled, bus_reset_work will 
> > unmask the interrupt after servicing the event, so future interrupts 
> > will be caught as desired.
> > 
> > As a side effect to this change, OHCI_PARAM_DEBUG_BUSRESETS can now be 
> > enabled through sysfs in addition to during initial module loading. 
> > However, when enabled through sysfs, logging of bus reset interrupts will 
> > be effective only starting with the second bus reset, after 
> > bus_reset_work has executed.
> > 
> > Signed-off-by: Adam Goldman <adamg@pobox.com>
> > ---
> > 
> > --- linux-6.8-rc1.orig/drivers/firewire/ohci.c	2024-01-21 14:11:32.000000000 -0800
> > +++ linux-6.8-rc1/drivers/firewire/ohci.c	2024-03-12 01:15:10.000000000 -0700
> > @@ -2060,6 +2060,8 @@ static void bus_reset_work(struct work_s
> >  
> >  	ohci->generation = generation;
> >  	reg_write(ohci, OHCI1394_IntEventClear, OHCI1394_busReset);
> > +	if (param_debug & OHCI_PARAM_DEBUG_BUSRESETS)
> > +		reg_write(ohci, OHCI1394_IntMaskSet, OHCI1394_busReset);
> >  
> >  	if (ohci->quirks & QUIRK_RESET_PACKET)
> >  		ohci->request_generation = generation;
> > @@ -2125,12 +2127,14 @@ static irqreturn_t irq_handler(int irq,
> >  		return IRQ_NONE;
> >  
> >  	/*
> > -	 * busReset and postedWriteErr must not be cleared yet
> > +	 * busReset and postedWriteErr events must not be cleared yet
> >  	 * (OHCI 1.1 clauses 7.2.3.2 and 13.2.8.1)
> >  	 */
> >  	reg_write(ohci, OHCI1394_IntEventClear,
> >  		  event & ~(OHCI1394_busReset | OHCI1394_postedWriteErr));
> >  	log_irqs(ohci, event);
> > +	if (event & OHCI1394_busReset)
> > +		reg_write(ohci, OHCI1394_IntMaskClear, OHCI1394_busReset);
> >  
> >  	if (event & OHCI1394_selfIDComplete)
> >  		queue_work(selfid_workqueue, &ohci->bus_reset_work);
> 
> Thanks for the patch. I pushed topic branch[1] for it, since I'm
> considering about whether to send it to stable and longterm releases.
> 
> I had realized that the debug=8 for firewire-ohci module provides tons
> of logs triggering by the irq handler, since the irq for bus reset is
> not unmasked, so I rely on selfID events when debugging bus-reset. I have
> few objections to the change.
> 
> My concern is how much invasive it is. The unmasking is kept until
> bus_reset_work() is executed in the workqueue. When considering about
> the delay of workqueue (since it is a kind of schedulable task), many
> irq events for bus reset is potentially skipped from the logging. Of
> course, it is the aim of change.
> 
> Let me take more time to evaluate the change, but I'm willing to send it
> to upstream until -rc3 or -rc4, at least, if receiving no objections
> from the others.
> 
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/ieee1394/linux1394.git/log/?h=v6.9-firewire-mask-bus-reset-event-during-handling

As long as I tested, the patch is enough good to suppress the spurious
IRQ event. I'll send it to mainline in this weekend.

I sent an additional patch[1] to handle the bus-reset event at the first
time. I'd like you to review and test it as well, especially under your
environment in which 1394:1995 and 1394a phys exist.

[1] https://lore.kernel.org/lkml/20240401121200.220013-1-o-takashi@sakamocchi.jp/


Thanks

Takashi Sakamoto