linux-kernel - Re: question about arch/arm/mach-s3c24xx/irq.c

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Date:	Sun, 24 Feb 2013 16:11:12 +0100
From:	Heiko Stübner <heiko@...ech.de>
To:	Julia Lawall <julia.lawall@...6.fr>
Cc:	"Russell King - ARM Linux" <linux@....linux.org.uk>,
	ben-linux@...ff.org, kgene.kim@...sung.com,
	linux-arm-kernel@...ts.infradead.org,
	linux-samsung-soc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: question about arch/arm/mach-s3c24xx/irq.c

Am Sonntag, 24. Februar 2013, 14:39:45 schrieb Julia Lawall:
> [Adding the person who introduced the code]
> 
> On Sun, 24 Feb 2013, Russell King - ARM Linux wrote:
> > On Sun, Feb 24, 2013 at 12:45:11PM +0100, Julia Lawall wrote:
> > > The function s3c24xx_irq_map in arch/arm/mach-s3c24xx/irq.c contains
> > > the
> > > 
> > > code:
> > >                 parent_irq_data =
> > >                 &parent_intc->irqs[irq_data->parent_irq];
> > > 		
> > > 		if (!irq_data) {
> > > 		
> > >                         pr_err("irq-s3c24xx: no irq data found for
> > >                         hwirq %lu\n",
> > >                         
> > >                                hw);
> > >                         
> > >                         goto err;
> > > 		
> > > 		}
> > > 
> > > At this point irq_data has already been tested, so the null test on
> > > irq_data does not look correct.  But I wonder if parent_irq_data could
> > > ever be null here?
> > 
> > That would be really obscure - because that would require parent_intc to
> > be a "negative" pointer (to counter-act the indexing by
> > irq_data->parent_irq).  So it looks to me like the above is redundant.
> 
> Even at its original definition irq_data seems unlikely to be NULL:
> 
>         struct s3c_irq_intc *intc = h->host_data;
> 	struct s3c_irq_data *irq_data = &intc->irqs[hw];
> 	...
> 	if (!irq_data) {
>                 pr_err("irq-s3c24xx: no irq data found for hwirq %lu\n",
> hw); return -EINVAL;
>         }
> 
> That is, it could be an invalid value, but whether it actually hits 0
> would seem to depend on the value hw?
> 
> Heiko, is NULL really a possibility?

The test you quoted is of course wrong ... it would need to test 
parent_irq_data. But you're also right that the test is not necessary at all.

All the s3c_irq_data arrays used always contain 32 entries to reach all bits 
of the register (which is used differently on each SoC). So if we have found 
the parent_intc at all, it should contain a 32 entries array of irq_data 
structs, so no need to test for the existence of the individual array element.


And now that I look at it, I also see another glitch. The code tests for 
parent_irq != 0, which of course won't work if the parent_irq is the 0-hwirq 
of the parent controller.
The only SoC using such a mapping is the s3c2412 [0], which explains why I 
haven't been bitten by this myself.


Heiko


[0] http://www.mail-archive.com/linux-samsung-
soc@...r.kernel.org/msg15709.html
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