Re: [PATCH v2 1/2] driver core: platform: Clarify that IRQ 0 is invalid

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, May 04, 2020 at 01:08:22PM -0500, Bjorn Helgaas wrote:
> On Sat, May 02, 2020 at 08:15:37AM +0200, Greg Kroah-Hartman wrote:
> > On Fri, May 01, 2020 at 05:40:41PM -0500, Bjorn Helgaas wrote:
> > > From: Bjorn Helgaas <bhelgaas@google.com>
> > > 
> > > These interfaces return a negative error number or an IRQ:
> > > 
> > >   platform_get_irq()
> > >   platform_get_irq_optional()
> > >   platform_get_irq_byname()
> > >   platform_get_irq_byname_optional()
> > > 
> > > The function comments suggest checking for error like this:
> > > 
> > >   irq = platform_get_irq(...);
> > >   if (irq < 0)
> > >     return irq;
> > > 
> > > which is what most callers (~900 of 1400) do, so it's implicit that IRQ 0
> > > is invalid.  But some callers check for "irq <= 0", and it's not obvious
> > > from the source that we never return an IRQ 0.
> > > 
> > > Make this more explicit by updating the comments to say that an IRQ number
> > > is always non-zero and adding a WARN() if we ever do return zero.  If we do
> > > return IRQ 0, it likely indicates a bug in the arch-specific parts of
> > > platform_get_irq().
> > 
> > I worry about adding WARN() as there are systems that do panic_on_warn()
> > and syzbot trips over this as well.  I don't think that for this issue
> > it would be a problem, but what really is this warning about that
> > someone could do anything with?
> > 
> > Other than that minor thing, this looks good to me, thanks for finally
> > clearing this up.
> 
> What I'm concerned about is an arch that returns 0.  Most drivers
> don't check for 0 so they'll just try to use it, and things will fail
> in some obscure way.  My assumption is that if there really is no IRQ,
> we should return -ENOENT or similar instead of 0.
> 
> I could be convinced that it's not worth warning about at all, or we
> could do something like the following:
> 
> diff --git a/drivers/base/platform.c b/drivers/base/platform.c
> index 084cf1d23d3f..4afa5875e14d 100644
> --- a/drivers/base/platform.c
> +++ b/drivers/base/platform.c
> @@ -220,7 +220,11 @@ int platform_get_irq_optional(struct platform_device *dev, unsigned int num)
>  	ret = -ENXIO;
>  #endif
>  out:
> -	WARN(ret == 0, "0 is an invalid IRQ number\n");
> +	/* Returning zero here is likely a bug in the arch IRQ code */
> +	if (ret == 0) {
> +		pr_warn("0 is an invalid IRQ number\n");
> +		dump_stack();
> +	}
>  	return ret;
>  }
>  EXPORT_SYMBOL_GPL(platform_get_irq_optional);
> @@ -312,7 +316,11 @@ static int __platform_get_irq_byname(struct platform_device *dev,
>  
>  	r = platform_get_resource_byname(dev, IORESOURCE_IRQ, name);
>  	if (r) {
> -		WARN(r->start == 0, "0 is an invalid IRQ number\n");
> +		/* Returning zero here is likely a bug in the arch IRQ code */
> +		if (r->start == 0) {
> +			pr_warn("0 is an invalid IRQ number\n");
> +			dump_stack();
> +		}
>  		return r->start;
>  	}
>  

I like that, but you said this is something that the platform people
should only see when bringing up a new system, so maybe the WARN() is
fine.  It's not user-triggerable, so your original is ok.

sorry for the noise,

greg k-h