Re: [PATCH 1/9] irqdomain: Reimplement irq_linear_revmap() with irq_find_mapping()

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 06/04/2021 à 11:35, Marc Zyngier a écrit :
> irq_linear_revmap() is supposed to be a fast path for domain
> lookups, but it only exposes low-level details of the irqdomain
> implementation, details which are better kept private.

Can you elaborate with more details ?

> 
> The *overhead* between the two is only a function call and
> a couple of tests, so it is likely that noone can show any
> meaningful difference compared to the cost of taking an
> interrupt.

Do you have any measurement ?

Can you make the "likely" a certitude ?

> 
> Reimplement irq_linear_revmap() with irq_find_mapping()
> in order to preserve source code compatibility, and
> rename the internal field for a measure.

This is in complete contradiction with commit https://github.com/torvalds/linux/commit/d3dcb436

At that time, irq_linear_revmap() was less complex than what irq_find_mapping() is today, and 
nevertheless it was considered worth restoring in as a fast path. What has changed since then ?

Can you also explain the reason for the renaming of "linear_revmap" into "revmap" ? What is that 
"measure" ?

> 
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>   include/linux/irqdomain.h | 22 +++++++++-------------
>   kernel/irq/irqdomain.c    |  6 +++---
>   2 files changed, 12 insertions(+), 16 deletions(-)
> 
> diff --git a/include/linux/irqdomain.h b/include/linux/irqdomain.h
> index 33cacc8af26d..b9600f24878a 100644
> --- a/include/linux/irqdomain.h
> +++ b/include/linux/irqdomain.h
> @@ -154,9 +154,9 @@ struct irq_domain_chip_generic;
>    * Revmap data, used internally by irq_domain
>    * @revmap_direct_max_irq: The largest hwirq that can be set for controllers that
>    *                         support direct mapping
> - * @revmap_size: Size of the linear map table @linear_revmap[]
> + * @revmap_size: Size of the linear map table @revmap[]
>    * @revmap_tree: Radix map tree for hwirqs that don't fit in the linear map
> - * @linear_revmap: Linear table of hwirq->virq reverse mappings
> + * @revmap: Linear table of hwirq->virq reverse mappings
>    */
>   struct irq_domain {
>   	struct list_head link;
> @@ -180,7 +180,7 @@ struct irq_domain {
>   	unsigned int revmap_size;
>   	struct radix_tree_root revmap_tree;
>   	struct mutex revmap_tree_mutex;
> -	unsigned int linear_revmap[];
> +	unsigned int revmap[];
>   };
>   
>   /* Irq domain flags */
> @@ -396,24 +396,20 @@ static inline unsigned int irq_create_mapping(struct irq_domain *host,
>   	return irq_create_mapping_affinity(host, hwirq, NULL);
>   }
>   
> -
>   /**
> - * irq_linear_revmap() - Find a linux irq from a hw irq number.
> + * irq_find_mapping() - Find a linux irq from a hw irq number.
>    * @domain: domain owning this hardware interrupt
>    * @hwirq: hardware irq number in that domain space
> - *
> - * This is a fast path alternative to irq_find_mapping() that can be
> - * called directly by irq controller code to save a handful of
> - * instructions. It is always safe to call, but won't find irqs mapped
> - * using the radix tree.
>    */
> +extern unsigned int irq_find_mapping(struct irq_domain *host,
> +				     irq_hw_number_t hwirq);
> +
>   static inline unsigned int irq_linear_revmap(struct irq_domain *domain,
>   					     irq_hw_number_t hwirq)
>   {
> -	return hwirq < domain->revmap_size ? domain->linear_revmap[hwirq] : 0;
> +	return irq_find_mapping(domain, hwirq);
>   }
> -extern unsigned int irq_find_mapping(struct irq_domain *host,
> -				     irq_hw_number_t hwirq);
> +
>   extern unsigned int irq_create_direct_mapping(struct irq_domain *host);
>   extern int irq_create_strict_mappings(struct irq_domain *domain,
>   				      unsigned int irq_base,
> diff --git a/kernel/irq/irqdomain.c b/kernel/irq/irqdomain.c
> index d10ab1d689d5..dfa716305ea9 100644
> --- a/kernel/irq/irqdomain.c
> +++ b/kernel/irq/irqdomain.c
> @@ -486,7 +486,7 @@ static void irq_domain_clear_mapping(struct irq_domain *domain,
>   				     irq_hw_number_t hwirq)
>   {
>   	if (hwirq < domain->revmap_size) {
> -		domain->linear_revmap[hwirq] = 0;
> +		domain->revmap[hwirq] = 0;
>   	} else {
>   		mutex_lock(&domain->revmap_tree_mutex);
>   		radix_tree_delete(&domain->revmap_tree, hwirq);
> @@ -499,7 +499,7 @@ static void irq_domain_set_mapping(struct irq_domain *domain,
>   				   struct irq_data *irq_data)
>   {
>   	if (hwirq < domain->revmap_size) {
> -		domain->linear_revmap[hwirq] = irq_data->irq;
> +		domain->revmap[hwirq] = irq_data->irq;
>   	} else {
>   		mutex_lock(&domain->revmap_tree_mutex);
>   		radix_tree_insert(&domain->revmap_tree, hwirq, irq_data);
> @@ -920,7 +920,7 @@ unsigned int irq_find_mapping(struct irq_domain *domain,
>   
>   	/* Check if the hwirq is in the linear revmap. */
>   	if (hwirq < domain->revmap_size)
> -		return domain->linear_revmap[hwirq];
> +		return domain->revmap[hwirq];
>   
>   	rcu_read_lock();
>   	data = radix_tree_lookup(&domain->revmap_tree, hwirq);
>