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Date:   Thu, 2 Apr 2020 14:25:10 -0300
From:   Marcelo Schmitt <marcelo.schmitt1@gmail.com>
To:     Lars-Peter Clausen <lars@metafoo.de>
Cc:     linux-kernel@vger.kernel.org, tglx@linutronix.de
Subject: Re: [RFC] genirq: prevent allocated_irqs from being smaller than
 NR_IRQS
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On 04/02, Lars-Peter Clausen wrote:
> On 4/2/20 5:08 PM, Marcelo Schmitt wrote:
> > Hi,
> > 
> > I was trying to understand IRQ initialization when suddenly got
> > intrigued about the declaration of the "allocated_irqs" bitmap at
> > kernel/irq/irqdesc.c. The size of allocated_irqs is defined by
> > IRQ_BITMAP_BITS, which in turn is passed to BITS_TO_LONGS to calculate
> > the actual number of IRQs the system may have. If I got it right, there
> > should be one entry at allocated_irqs for each possible IRQ line. At
> > kernel/irq/internals.h, IRQ_BITMAP_BITS is defined to be NR_IRQS (or
> > NR_IRQS plus a high constant in the case of sparse IRQs), which most
> > architectures seem to define as being the actual number of IRQs a board
> > has.
> > 
> > #ifdef CONFIG_SPARSE_IRQ
> > # define IRQ_BITMAP_BITS (NR_IRQS + 8196)
> > #else
> > # define IRQ_BITMAP_BITS NR_IRQS
> > #endif
> > 
> > The thing I'm troubled about is that BITS_TO_LONGS divides
> > IRQ_BITMAP_BITS by sizeof(long) * 8, which makes it possible for the
> > size of allocated_irqs to be smaller than NR_IRQS.
> > 
> > For instance, if !CONFIG_SPARSE_IRQ, sizeof(long) == 8, and NR_IRQS is
> > defined as 16, then IRQ_BITMAP_BITS would be equal to
> > (16 + 64 - 1)/64 = 1. Even if CONFIG_SPARSE_IRQ is defined, a device
> > with a large number of IRQ lines would end up with a small bitmap for
> > allocated_irqs.
> > 
> > I thought NR_IRQS would be multiplied by the number of bits it uses.
> > Something like:
> > 
> > #ifdef CONFIG_SPARSE_IRQ
> > # define IRQ_BITMAP_BITS (NR_IRQS * BITS_PER_TYPE(long) + 8196)
> > #else
> > # define IRQ_BITMAP_BITS (NR_IRQS * BITS_PER_TYPE(long))
> > #endif
> > 
> > Anyhow, IRQ_BITMAP_BITS is also used to limit the maximum number of IRQs
> > at irqdesc.c. If my understanding of nr_irqs is correct, it would make
> > sense to change some sanity checks at early_irq_init() too.
> > 
> > Does anyone mind giving me some advice on how allocated_irqs is
> > initialized with a suitable size to support the number of interrupt
> > lines a board may have?
> 
> Maybe I'm missing something, but allocated_irqs is a bitmap. This means each
> bit corresponds to one IRQ. if sizeof(long) is 8 and allocated_irqs is sized
> to be one long that means it is large enough for 64 IRQs.

Probably that's what I was missing about the allocated_irqs use.
This explains a lot.

Many thanks,

Marcelo

> 
> - Lars
>