On Tue, 19 Mar 2013, Daniel Vetter wrote:
> For reference below the updated commit message.
>
> Cheers, Daniel
>
> Author: Jiri Kosina <jkosina@xxxxxxx>
> Date: Tue Mar 19 09:56:57 2013 +0100
>
> drm/i915: stop using GMBUS IRQs on Gen4 chips
>
> Commit 28c70f162 ("drm/i915: use the gmbus irq for waits") switched to
> using GMBUS irqs instead of GPIO bit-banging for chipset generations 4
> and above.
>
> It turns out though that on many systems this leads to spurious interrupts
> being generated, long after the register write to disable the IRQs has been
> issued.
>
> Typically this results in the spurious interrupt source getting
> disabled:
>
> [ 9.636345] irq 16: nobody cared (try booting with the "irqpoll" option)
> [ 9.637915] Pid: 4157, comm: ifup Tainted: GF
> 3.9.0-rc2-00341-g0863702 #422
> [ 9.639484] Call Trace:
> [ 9.640731] <IRQ> [<ffffffff8109b40d>] __report_bad_irq+0x1d/0xc7
> [ 9.640731] [<ffffffff8109b7db>] note_interrupt+0x15b/0x1e8
> [ 9.640731] [<ffffffff810999f7>] handle_irq_event_percpu+0x1bf/0x214
> [ 9.640731] [<ffffffff81099a88>] handle_irq_event+0x3c/0x5c
> [ 9.640731] [<ffffffff8109c139>] handle_fasteoi_irq+0x7a/0xb0
> [ 9.640731] [<ffffffff8100400e>] handle_irq+0x1a/0x24
> [ 9.640731] [<ffffffff81003d17>] do_IRQ+0x48/0xaf
> [ 9.640731] [<ffffffff8142f1ea>] common_interrupt+0x6a/0x6a
> [ 9.640731] <EOI> [<ffffffff8142f952>] ? system_call_fastpath+0x16/0x1b
> [ 9.640731] handlers:
> [ 9.640731] [<ffffffffa000d771>] usb_hcd_irq [usbcore]
> [ 9.640731] [<ffffffffa0306189>] yenta_interrupt [yenta_socket]
> [ 9.640731] Disabling IRQ #16
>
> The really curious thing is now that irq 16 is _not_ the interrupt for
> the i915 driver when using MSI, but it _is_ the interrupt when not
> using MSI. So by all indications it seems like gmbus is able to
> generate a legacy (shared) interrupt in MSI mode on some
> configurations. I've tried to reproduce this and the differentiating
> thing seems to be that on unaffected systems no other device uses irq
> 16 (which seems to be the non-MSI intel gfx interrupt on all gm45).
That might be misleading. It's possible that the erroneous IRQs _are_
being issued but you're simply not aware of them. If the kernel thinks
that no device is using IRQ 16 then it will leave that IRQ disabled.
Alan Stern
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