Message-ID: <1304484421.3255.150.camel@x201>
Date:	Tue, 03 May 2011 22:47:01 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Dave Airlie <airlied@...hat.com>
Cc:	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 2/2] drm/nouveau: Check that the device is enabled
 before processing interrupt

On Wed, 2011-05-04 at 14:22 +1000, Dave Airlie wrote:
> On Tue, 2011-05-03 at 22:18 -0600, Alex Williamson wrote:
> > On Wed, 2011-05-04 at 13:50 +1000, Dave Airlie wrote:
> > > On Mon, May 2, 2011 at 10:49 AM, Alex Williamson
> > > <alex.williamson@...hat.com> wrote:
> > > > We're likely to be sharing an interrupt line with other devices,
> > > > which means our handler might get called after we've turned off
> > > > the device via vga switcheroo.  This can lead to all sorts of
> > > > badness, like nv04_fifo_isr() spewing "PFIFO still angry after
> > > > 100 spins, halt" to the console before the system enters a hard
> > > > hang.
> > > >
> > > > We can avoid this by simply checking if the device is still
> > > > enabled before processing an interrupt.  To avoid races, flush
> > > > any inflight interrupts using synchronize_irq().  Note that
> > > > since pci_intx() is called after pci_save_state(),
> > > > pci_restore_state() will automatically re-enable INTx.
> > > 
> > > I still think we should just need the synchronize_irq followed by a
> > > check in the irq handler for all fs,
> > > 
> > > or is there a race there I'm missing?
> > 
> > The synchronize_irq by itself doesn't guarantee anything.  The irq
> > handler could be immediately started on another CPU once that returns
> > and be well past the first device read before we make it far enough
> > through pci_set_power_state that the device becomes unresponsive.  Can
> > we guarantee that first device read in the interrupt handler will always
> > be 0 or -1 in the suspend path?  Even as the last milliamperes of charge
> > drain out of the device?
> 
> It should always be a valid irq or 0xffffffff. It got nothing to do with
> milliamperes of charge, and all to do with the PCI BAR decodes being
> turned off.

Unfortunately this depends on the platform behavior for a master abort.
But maybe this bring up another issue; we really want to make sure we're
turning off decode after we've flushed any inflight interrupts.  We
can't guarantee that w/o some kind of synchronization and the device
being enabled is probably too late... are you sure you don't want to
uninstall the interrupt handler? ;)

> > By adding the device enabled check in the interrupt handler, disabling
> > the device, then calling synchronize_irq, we guarantee that the entire
> > interrupt handler path is not being executed and won't be executed again
> > until we re-enable the device.  It does seem a bit odd, but how many
> > other devices in the system are entirely powered off, with a driver
> > attached and interrupt handler registered while the system is still
> > humming along?  Thanks,
> 
> The theory is lots. OLPC does this sort of things for its breakfast I'd
> have thought.
> 
> which is why I still think we are missing something, when we D3 the
> device it should be the same as suspend/resume cycle pretty much,

Except the whole system goes down for a suspend/resume and we don't
typically have to worry about stray interrupt during the down time.  If
our vga switcheroo handler is doing the right thing, we're not only
going to D3, we're entirely removing power from the device.  Some
platforms (probably not ones we care about for switcheroo, but
nonetheless) won't put up with a master abort on the bus that would be
caused by reading from an effectively non-existent device.  Thanks,

Alex

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