linux-kernel - Re: [PATCH] x86/apic: Update virtual irq base for DT/OF based system as well

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Date:   Wed, 21 Aug 2019 17:31:20 +0800
From:   "Tanwar, Rahul" <rahul.tanwar@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     mingo@...hat.com, bp@...en8.de, hpa@...or.com, tony.luck@...el.com,
        x86@...nel.org, andriy.shevchenko@...el.com, alan@...ux.intel.com,
        rppt@...ux.ibm.com, linux-kernel@...r.kernel.org,
        qi-ming.wu@...el.com, cheol.yong.kim@...el.com,
        rahul.tanwar@...el.com
Subject: Re: [PATCH] x86/apic: Update virtual irq base for DT/OF based system
 as well

On 21/8/2019 4:34 PM, Thomas Gleixner wrote:

> Secondly, this link is irrelevant. ioapic_dynirq_base has nothing to do
> with virtual IRQ number 0. It's a boundary for the dynamic allocation of
> virtual interrupt numbers so that the core allocator does not pick
> interrupts out of the IOAPIC's fixed interrupt number space.
>
> This can be legitimately 0 when IOAPIC is not enabled at all.
>
> Can you please explain what kind of problem you were seing and what this
> really fixes?

The problem is that device tree infrastructure considers 0 IRQ value as 
invalid/error

value whereas for ACPI, 0 is a valid value. Without this change, the 
problem that we

see is that the first driver using of_irq_get_xx() or its variants fails 
because of 0 IRQ

number. With this change, allocated IRQ number is never 0 so it works ok.