Re: [Intel-gfx] [PATCH] drm/i915: Remove unused IRQ chip data of HDMI LPE audio

[Ville Syrjälä]

On Mon, Dec 11, 2017 at 08:33:33AM +0000, Anand, Jerome wrote:
> 
> 
> > -----Original Message-----
> > From: Thomas Gleixner [mailto:tglx@xxxxxxxxxxxxx]
> > Sent: Saturday, December 9, 2017 4:22 AM
> > To: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> > Cc: Chen, Augustine <augustine.chen@xxxxxxxxx>; intel-
> > gfx@xxxxxxxxxxxxxxxxxxxxx; alsa-devel@xxxxxxxxxxxxxxxx; Anand, Jerome
> > <jerome.anand@xxxxxxxxx>; Bossart, Pierre-louis <pierre-
> > louis.bossart@xxxxxxxxx>; tiwai@xxxxxxx; Ingo Molnar <mingo@xxxxxxxxxx>;
> > H. Peter Anvin <hpa@xxxxxxxxx>; Jiang Liu <jiang.liu@xxxxxxxxxxxxxxx>; Juergen
> > Gross <jgross@xxxxxxxx>; Dou Liyang <douly.fnst@xxxxxxxxxxxxxx>; linux-
> > kernel@xxxxxxxxxxxxxxx
> > Subject: Re: [Intel-gfx] [PATCH] drm/i915: Remove unused IRQ chip data of
> > HDMI LPE audio
> > 
> > On Fri, 8 Dec 2017, Ville Syrjälä wrote:
> > 
> > > On Fri, Dec 08, 2017 at 05:33:23PM +0800, Augustine.Chen wrote:
> > > > The chip data of HDMI LPE audio is set to drm_i915_private which is
> > > > not consistent with the expectation by x86 APIC driver.
> > >
> > > Hmm. Why is the apic code looking at data for an irq chip it hasn't
> > > created?
> > >
> 
> apic code expects an irq domain to be place as generic approach.
> 
> > > Do we need something like
> > > - dev_priv->lpe_audio.irq = irq_alloc_desc(0);
> > > + dev_priv->lpe_audio.irq = irq_alloc_desc(-1);
> > 
> > #define irq_alloc_desc(node)
> > 
> > So instead of handing in node 0 you hand in node -1 which is NUMA_NO_NODE
> > 

Ah. I misread the macros. So we already pass irq=-1.

> 
> Agree - am not sure whether it will make any difference.
> 
> > > That *looks* more correct to me based on a cursory glance at the x86
> > > code, but I didn't trawl very deeply.
> > 
> > The x86 core cares not at all about interrupt chips which are created in a driver
> > and not connected to an actual apic/ioapic/msi interrupt. This interrupt chip is
> > its own thing as we have others in GPIO etc.
> > 
> > > > In the case of not enabling CONFIG_CPUMASK_OFFSTACK, this would
> > > > cause kernel panic while doing CPU hotplug.
> > 
> > And why so? Surely not because you set irq_chip_data. That's really no
> > explanation at all.
> > 
> 
> Ideally, I feel there needs to be an irq domain for mapping the irq numbers with hwirq.
> It is not created as part of the hdmi lpe audio bridge.
> Since the logic to mask/unmask lpe audio interrupts is removed, there is no need of the
> Chip data or creation of the domain now.

There is no need right now. But there might be a need in the future.
LPE audio isn't even the only piece of hardware whose irq goes through
the i915 display engine (there's also the ISP and VED). So I would
much prefer a proper solution instead of sweeping the problem under
the rug.

-- 
Ville Syrjälä
Intel OTC