linux-kernel - Re: [Patch V4 12/42] x86, ioapic: kill static variable nr_irqs

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Date:	Thu, 12 Jun 2014 12:58:37 +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,
	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>
Subject: Re: [Patch V4 12/42] x86, ioapic: kill static variable nr_irqs_gsi

On Tue, 10 Jun 2014, Jiang Liu wrote:

> Hi Thomas,
> 	This piece of code is inherited from current IOAPIC driver
> and I think it's a workaround for some weird platforms.
> 	For normal platforms with both 8259A and IOAPIC controllers,
> legacy ISA IRQs should be connected to both 8259A and IOAPIC pins
> (ignore timer and cascade IRQs for simplicity). According to comments
>  in current kernel, there are some platforms on which:
> 1) some ISA IRQs are only connected to 8259A controllers.
> 2) the corresponding IOAPIC pins are connected to some non-ISA IRQs.
> 	For such platforms, IRQ0-15 are used for ISA IRQs and another
> 16 IRQs just above gsi_top are reserved for IOAPIC pins 0-15 which
> are connected to non-ISA IRQs.
> 	I have no real experience with such a platform, but just
> guessing possible cases according to kernel comments and "Multiple
> Processor Specification". Please look at these two pictures for quick
> reference.
> http://www.manualslib.com/manual/77733/Intel-Multiprocessor.html?page=31#manual
> http://www.manualslib.com/manual/77733/Intel-Multiprocessor.html?page=63#manual

Duh. I completely forgot about the 82489 mess.

We probably want a comment somewhere why we have this gsi + legacy
thing.

Thanks,

	tglx
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