linux-kernel - Re: [PATCH v8 5/8] genirq/irq: introduce msi_doorbell

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Date:	Wed, 4 May 2016 13:57:30 +0100
From:	Marc Zyngier <marc.zyngier@....com>
To:	Eric Auger <eric.auger@...aro.org>, eric.auger@...com,
	robin.murphy@....com, alex.williamson@...hat.com,
	will.deacon@....com, joro@...tes.org, tglx@...utronix.de,
	jason@...edaemon.net, christoffer.dall@...aro.org,
	linux-arm-kernel@...ts.infradead.org
Cc:	patches@...aro.org, linux-kernel@...r.kernel.org,
	Bharat.Bhushan@...escale.com, pranav.sawargaonkar@...il.com,
	p.fedin@...sung.com, iommu@...ts.linux-foundation.org,
	Jean-Philippe.Brucker@....com, julien.grall@....com
Subject: Re: [PATCH v8 5/8] genirq/irq: introduce msi_doorbell_info

On 28/04/16 09:22, Eric Auger wrote:
> The purpose is to be able to retrieve the MSI doorbells of an irqchip.
> This is now needed since on some platforms those doorbells must be
> iommu mapped (in case the MSIs transit through an IOMMU that do not
> bypass those transactions).
> 
> The assumption is there is a maximum of one doorbell region per cpu.
> 
> A doorbell region is characterized by its physical address base, size and
> IOMMU protection flag. Those 2 last characteristics are shared among all
> doorbells.
> 
> irq_chip msi_doorbell_info callback enables to retrieve the doorbells of
> the irqchip.
> 
> Signed-off-by: Eric Auger <eric.auger@...aro.org>
> 
> ---
> v7 -> v8:
> - size and prot now are shared among all doorbells
> - doorbells now directly points to a percpu phys_addr_t
> 
> v7: creation
> ---
>  include/linux/irq.h | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/irq.h b/include/linux/irq.h
> index c4de623..5dbb26d 100644
> --- a/include/linux/irq.h
> +++ b/include/linux/irq.h
> @@ -312,6 +312,19 @@ static inline irq_hw_number_t irqd_to_hwirq(struct irq_data *d)
>  	return d->hwirq;
>  }
>  
> +/*
> + * Describe all the MSI doorbell regions for an irqchip.
> + * A single doorbell region per cpu is assumed.
> + * In case a single doorbell is supported for the whole irqchip,
> + * the region is described in as cpu #0's one
> + */
> +struct irq_chip_msi_doorbell_info {
> +	phys_addr_t __percpu *percpu_doorbells; /* per cpu base address */

Here's an idea: you could turn this field into a union:

	union {
		phys_addr_t __percpu *percpu_doorbells;
		phys_addr_t global_doorbell;
	};

> +	size_t size;				/* size of a each doorbell */
> +	int prot;				/* iommu protection flag */
> +	int nb_doorbells;

And this can be turned into a boolean:

	bool doorbell_is_percpu;

This allows you to avoid allocating a bunch of percpu doorbells for a
MSI controller that only has a global one. And the number of doorbells
is always either 1 or the number of CPUs anyway.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...