linux-kernel - Re: [PATCH 1/7] genirq/msi: Add a new field in msi

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Message-ID: <1bcdb72f-2116-756b-b74d-a48f89870782@arm.com>
Date:   Tue, 23 Apr 2019 11:51:48 +0100
From:   Julien Grall <julien.grall@....com>
To:     Marc Zyngier <marc.zyngier@....com>, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org
Cc:     logang@...tatee.com, douliyangs@...il.com,
        miquel.raynal@...tlin.com, jason@...edaemon.net,
        tglx@...utronix.de, joro@...tes.org, robin.murphy@....com,
        bigeasy@...utronix.de, linux-rt-users@...r.kernel.org
Subject: Re: [PATCH 1/7] genirq/msi: Add a new field in msi_desc to store an
 IOMMU cookie

On 4/23/19 11:23 AM, Marc Zyngier wrote:
> Hi Julien,

Hi Marc,

> On 18/04/2019 18:26, Julien Grall wrote:
>> When an MSI doorbell is located downstream of an IOMMU, it is required
>> to swizzle the physical address with an appropriately-mapped IOVA for any
>> device attached to one of our DMA ops domain.
>>
>> At the moment, the allocation of the mapping may be done when composing
>> the message. However, the composing may be done in non-preemtible
>> context while the allocation requires to be called from preemptible
>> context.
>>
>> A follow-up patch will split the current logic in two functions
>> requiring to keep an IOMMU cookie per MSI.
>>
>> This patch introduces a new field in msi_desc to store an IOMMU cookie
>> when CONFIG_IOMMU_DMA is selected.
>>
>> Signed-off-by: Julien Grall <julien.grall@....com>
>> ---
>>   include/linux/msi.h | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/include/linux/msi.h b/include/linux/msi.h
>> index 7e9b81c3b50d..d7907feef1bb 100644
>> --- a/include/linux/msi.h
>> +++ b/include/linux/msi.h
>> @@ -77,6 +77,9 @@ struct msi_desc {
>>   	struct device			*dev;
>>   	struct msi_msg			msg;
>>   	struct irq_affinity_desc	*affinity;
>> +#ifdef CONFIG_IOMMU_DMA
>> +	const void			*iommu_cookie;
>> +#endif
>>   
>>   	union {
>>   		/* PCI MSI/X specific data */
>>
> 
> Given that this is the only member in this structure that is dependent
> on a config option, you could also add a couple of accessors that would
> do nothing when IOMMU_DMA is not selected (and use that in the DMA code).

I haven't seen any use of the helpers so far because the DMA code is 
also protected by IOMMU_DMA.

I can add the helpers in the next version if you see any use outside of 
the DMA code.

Cheers,

-- 
Julien Grall