linux-kernel - Re: [PATCH v2 09/10] genirq/irqdomain: fall back to default domain when creating hierarchy domain

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Date:   Tue, 19 Feb 2019 16:27:16 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     Thomas Bogendoerfer <tbogendoerfer@...e.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 09/10] genirq/irqdomain: fall back to default domain
 when creating hierarchy domain

On Tue, 19 Feb 2019 16:57:23 +0100
Thomas Bogendoerfer <tbogendoerfer@...e.de> wrote:

Hi Thomas,

> When creating hierarchy domains use irq_default_domain as parent, if no
> parent was given by the caller. This avoids adding helper code for
> querying the underlying platform irq domain.
> 
> Signed-off-by: Thomas Bogendoerfer <tbogendoerfer@...e.de>
> ---
>  kernel/irq/irqdomain.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/irq/irqdomain.c b/kernel/irq/irqdomain.c
> index 8b0be4bd6565..617c482d0778 100644
> --- a/kernel/irq/irqdomain.c
> +++ b/kernel/irq/irqdomain.c
> @@ -1021,7 +1021,10 @@ struct irq_domain *irq_domain_create_hierarchy(struct irq_domain *parent,
>  	else
>  		domain = irq_domain_create_tree(fwnode, ops, host_data);
>  	if (domain) {
> -		domain->parent = parent;
> +		if (parent)
> +			domain->parent = parent;
> +		else
> +			domain->parent = irq_default_domain;
>  		domain->flags |= flags;
>  	}
>  

I'm really not keen on this. The whole "default domain" made sense at a
distant point in time (when irqdomains were new and platform code was
blissfully ignoring it), but it really looks like a sore spot in the
hierarchy code, which assumes that you always know what you're building
your hierarchy on top of.

It also create a small issue in the sense that you can create a root
domain using irq_domain_create_hierarchy() by passing NULL as the
parent. With this patch, the new domain now points to the default one,
with unexpected consequences.

So let's come back to first principles: How comes you can't obtain the
parent domain at creation time? Because I'd rather give you a way to
retrieve it instead if this.

	M. (who wish he had an IP27!)
-- 
Without deviation from the norm, progress is not possible.