linux-kernel - Re: [Patch V3 19/37] x86, irq: introduce mechanisms to support dynamically allocate IRQ for IOAPIC

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Date:	Wed, 28 May 2014 23:08:33 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jiang Liu <jiang.liu@...ux.intel.com>
cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Grant Likely <grant.likely@...aro.org>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	Yinghai Lu <yinghai@...nel.org>, x86@...nel.org,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tony Luck <tony.luck@...el.com>,
	Joerg Roedel <joro@...tes.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	linux-acpi@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	linux-pm@...r.kernel.org
Subject: Re: [Patch V3 19/37] x86, irq: introduce mechanisms to support
 dynamically allocate IRQ for IOAPIC

On Wed, 28 May 2014, Jiang Liu wrote:
> On 2014/5/28 3:58, Thomas Gleixner wrote:
> > So you have these cases covered here:
> > 
> > 1) The ACPI case of secondary ioapics. You only have the strict 1:1
> >    mapping for the first ioapic
> > 
> > 2) The gsi < NR_IRQS_LEGACY case where you have two options:
> > 
> >     a) Let the core create a random virq number if ioapic_identity_map
> >        is 0
> > 
> >        ioapic_identity_map is only set by SFI and devicetree
> > 
> >        So in all other cases we fall into that code path for all
> >        legacy interrupts. So how is that supposed to work lets say for
> >        i8042 which has hardcoded irq 1 and 12?
> > 
> >        irq_create_mapping(1)
> >        
> > 	    hint = 1 % nr_irqs; --> 1
> > 	    virq = irq_alloc_desc_from(hint, of_node_to_nid(domain->of_node));
> > 
> > 	    This returns something >= 16, because the irq descriptors
> > 	    for 0-15 (LEGACY) are allocated already.
> > 
> >        The pin association works, but how is the i8042 driver supposed
> >        to figure out that it should request the virq >=16 which was
> >        created instead of the hardcoded 1 ?
> This is used to work around special non-ISA interrupts with GSI below
> NR_IRQS_LEGACY. The original code for the special case is:
> /*
>  * Provide an identity mapping of gsi == irq except on truly
>  * weird platforms that have non isa irqs in the first 16 gsis.
>  */
> return gsi >= NR_IRQS_LEGACY ? gsi : gsi_top + gsi;

That looks really, really wrong. What's wrong with assigning that irq
irq number on those platforms?

The weird stuff is SFI and devicetree, if I understand your code
correctly.

So if those platforms do not have actual legacy irqs, what's wrong
with giving out the legacy numbers?

> We have one path to handle ISA IRQs before calling
> alloc_irq_from_domain() as below:
>         if (idx >= 0 && test_bit(mp_irqs[idx].srcbus, mp_bus_not_pci))
>                 return mp_irqs[idx].srcbusirq;

Ok.
 
> >         /* We can't set this earlier, because we need to calibrate the timer */
> >         legacy_pic = &null_legacy_pic;
> I haven't figured out the story behind the comment yet:(

Sebastian gave some insight.
 
> > Why do we need strict mappings in the non ACPI case for all ioapic
> > pins? What's so different about ACPI? Or is this just to avoid
> > breaking the existing SFI/devicetree stuff. If that's the reason I'm
> > fine with it, but ...
> It's to avoid breaking SFI/intel_mid stuff. intel_mid assumes IRQ
> number equals to pin number and use pci_dev->irq to save both IRQ
> number and pin number.

Fair enough.

Thanks,

	tglx

 
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