linux-kernel - Is IRQ number 0 a valid IRQ ?

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Date:   Sat, 5 Oct 2019 12:47:17 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Is IRQ number 0 a valid IRQ ?

Hi Thomas,

This is something which I have been wondering for ever since there are
several places in the kernel where IRQ number 0 is treated as not being
valid (as no IRQ found mostly I guess). Where as other places do treat
IRQ number 0 as valid... ?

Some examples which treat IRQ 0 special:

drivers/base/platform.c: __platform_get_irq() :

         if (IS_ENABLED(CONFIG_OF_IRQ) && dev->dev.of_node) {
                 int ret;

                 ret = of_irq_get(dev->dev.of_node, num);
                 if (ret > 0 || ret == -EPROBE_DEFER)
                         return ret;
         }

Note if (ret > 0) not if (ret >= 0)

Other example: drivers/usb/dwc3/gadget.c: dwc3_gadget_get_irq() :

         if (!irq)
                 irq = -EINVAL;

So 2 questions:

1) Is this special handling of IRQ number 0 valid code, or just
mostly some leftover from older days when IRQ number 0 was maybe
special ?

2) Either way (*) I think we (I volunteer) should document this somewhere,
other then adding a note about this to the platform_get_irq docs any
other place where it would be good to specify this?

Regards,

Hans

*) Either IRQ number 0 is not special and then we need to stop the
cargo-culting of treating it special, or it is special and then we
need to document that.