Subject: Re: next/mmotm unbootable on G5: irqdomain

From: Benjamin Herrenschmidt <[email-masked]>
Date: Sun, 22 Jul 2012 23:09:14 +1000

On Sat, 2012-07-21 at 19:47 -0700, Hugh Dickins wrote:
> I have to revert the patch below from mmotm 2012-07-20-16-30 or
> next-20120720 in order to boot on the PowerPC G5: otherwise it
> freezes before switching to the framebuffer console - but I'm
> not certain where because that initial console doesn't scroll
> (there are mpic messages at bottom and at top of screen, probably
> later messages at the top but I don't know the sequence).

Remind me your G5 variant ? (/proc/cpuinfo will do). I'll have a look
tomorrow (and thanks for testing !).

Cheers,
Ben.

> Hugh
> 
> commit 94f036a1f242f98cc30700b7676c07270a9c5c27
> Author: Grant Likely <[email-masked]>
> Date:   Sun Jun 3 22:04:39 2012 -0700
> 
> irqdomain: eliminate slow-path revmap lookups
> 
> With the current state of irq_domain, the reverse map is always updated
> when new IRQs get mapped.  This means that the irq_find_mapping() function
> can be simplified to execute the revmap lookup functions unconditionally
> 
> This patch adds lookup functions for the revmaps that don't yet have one
> and removes the slow path lookup code path.
> 
> v8: Broke out unrelated changes into separate patches.  Rebased on Paul's irq
>     association patches.
> v7: Rebased to irqdomain/next for v3.4 and applied before the removal of 'hint'
> v6: Remove the slow path entirely.  The only place where the slow path
>     could get called is for a linear mapping if the hwirq number is larger
>     than the linear revmap size.  There shouldn't be any interrupt
>     controllers that do that.
> v5: rewrite to not use a ->revmap() callback.  It is simpler, smaller,
>     safer and faster to open code each of the revmap lookups directly into
>     irq_find_mapping() via a switch statement.
> v4: Fix build failure on incorrect variable reference.
> 
> Signed-off-by: Grant Likely <[email-masked]>
> Cc: Benjamin Herrenschmidt <[email-masked]>
> Cc: Thomas Gleixner <[email-masked]>
> Cc: Milton Miller <[email-masked]>
> Cc: Paul Mundt <[email-masked]>
> Cc: Rob Herring <[email-masked]>
> 
> diff --git a/kernel/irq/irqdomain.c b/kernel/irq/irqdomain.c
> index c0e638b..a9b810e 100644
> --- a/kernel/irq/irqdomain.c
> +++ b/kernel/irq/irqdomain.c
> @@ -686,16 +686,11 @@ EXPORT_SYMBOL_GPL(irq_dispose_mapping);
>   * irq_find_mapping() - Find a linux irq from an hw irq number.
>   * @domain: domain owning this hardware interrupt
>   * @hwirq: hardware irq number in that domain space
> - *
> - * This is a slow path, for use by generic code. It's expected that an
> - * irq controller implementation directly calls the appropriate low level
> - * mapping function.
>   */
>  unsigned int irq_find_mapping(struct irq_domain *domain,
>  			      irq_hw_number_t hwirq)
>  {
> -	unsigned int i;
> -	unsigned int hint = hwirq % nr_irqs;
> +	struct irq_data *data;
>  
>  	/* Look for default domain if nececssary */
>  	if (domain == NULL)
> @@ -703,22 +698,27 @@ unsigned int irq_find_mapping(struct irq_domain *domain,
>  	if (domain == NULL)
>  		return 0;
>  
> -	/* legacy -> bail early */
> -	if (domain->revmap_type == IRQ_DOMAIN_MAP_LEGACY)
> +	switch (domain->revmap_type) {
> +	case IRQ_DOMAIN_MAP_LEGACY:
>  		return irq_domain_legacy_revmap(domain, hwirq);
> -
> -	/* Slow path does a linear search of the map */
> -	if (hint == 0)
> -		hint = 1;
> -	i = hint;
> -	do {
> -		struct irq_data *data = irq_get_irq_data(i);
> +	case IRQ_DOMAIN_MAP_LINEAR:
> +		return irq_linear_revmap(domain, hwirq);
> +	case IRQ_DOMAIN_MAP_TREE:
> +		rcu_read_lock();
> +		data = radix_tree_lookup(&domain->revmap_data.tree, hwirq);
> +		rcu_read_unlock();
> +		if (data)
> +			return data->irq;
> +		break;
> +	case IRQ_DOMAIN_MAP_NOMAP:
> +		data = irq_get_irq_data(hwirq);
>  		if (data && (data->domain == domain) && (data->hwirq == hwirq))
> -			return i;
> -		i++;
> -		if (i >= nr_irqs)
> -			i = 1;
> -	} while(i != hint);
> +			return hwirq;
> +		break;
> +	}
> +
> +	WARN(1, "ERROR: irq revmap went horribly wrong. revmap_type=%i\n",
> +		domain->revmap_type);
>  	return 0;
>  }
>  EXPORT_SYMBOL_GPL(irq_find_mapping);
> @@ -728,32 +728,19 @@ EXPORT_SYMBOL_GPL(irq_find_mapping);
>   * @domain: domain owning this hardware interrupt
>   * @hwirq: hardware irq number in that domain space
>   *
> - * This is a fast path, for use by irq controller code that uses linear
> - * revmaps. It does fallback to the slow path if the revmap doesn't exist
> - * yet and will create the revmap entry with appropriate locking
> + * This is a fast path that can be called directly by irq controller code to
> + * save a handful of instructions.
>   */
>  unsigned int irq_linear_revmap(struct irq_domain *domain,
>  			       irq_hw_number_t hwirq)
>  {
> -	unsigned int *revmap;
> +	BUG_ON(domain->revmap_type != IRQ_DOMAIN_MAP_LINEAR);
>  
> -	if (WARN_ON_ONCE(domain->revmap_type != IRQ_DOMAIN_MAP_LINEAR))
> -		return irq_find_mapping(domain, hwirq);
> -
> -	/* Check revmap bounds */
> -	if (unlikely(hwirq >= domain->revmap_data.linear.size))
> -		return irq_find_mapping(domain, hwirq);
> -
> -	/* Check if revmap was allocated */
> -	revmap = domain->revmap_data.linear.revmap;
> -	if (unlikely(revmap == NULL))
> -		return irq_find_mapping(domain, hwirq);
> -
> -	/* Fill up revmap with slow path if no mapping found */
> -	if (unlikely(!revmap[hwirq]))
> -		revmap[hwirq] = irq_find_mapping(domain, hwirq);
> +	/* Check revmap bounds; complain if exceeded */
> +	if (WARN_ON(hwirq >= domain->revmap_data.linear.size))
> +		return 0;
>  
> -	return revmap[hwirq];
> +	return domain->revmap_data.linear.revmap[hwirq];
>  }
>  EXPORT_SYMBOL_GPL(irq_linear_revmap);
>  
> --
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