linux-kernel - Re: [PATCH v12 3/5] iommu/vt-d: Add support for IOMMU default DMA mode build options

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Date:   Tue, 15 Jun 2021 09:25:10 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>,
        John Garry <john.garry@...wei.com>, joro@...tes.org,
        will@...nel.org, dwmw2@...radead.org
Cc:     linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linuxarm@...wei.com, thunder.leizhen@...wei.com,
        chenxiang66@...ilicon.com
Subject: Re: [PATCH v12 3/5] iommu/vt-d: Add support for IOMMU default DMA
 mode build options

On 2021-06-15 08:26, Lu Baolu wrote:
> Hi John,
> 
> On 6/14/21 4:03 PM, John Garry wrote:
>> On 12/06/2021 03:14, Lu Baolu wrote:
>>> On 2021/6/11 20:20, John Garry wrote:
>>>> diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
>>>> index 2a71347611d4..4467353f981b 100644
>>>> --- a/drivers/iommu/Kconfig
>>>> +++ b/drivers/iommu/Kconfig
>>>> @@ -94,6 +94,7 @@ choice
>>>>       prompt "IOMMU default DMA mode"
>>>>       depends on IOMMU_DMA
>>>> +    default IOMMU_DEFAULT_LAZY if INTEL_IOMMU
>>>>       default IOMMU_DEFAULT_STRICT
>>>
>>> If two default values are different. Which one will be overridden?
>>
>> If I understand your question correctly, I think you are asking if 
>> both are set:
>> CONFIG_IOMMU_DEFAULT_LAZY=y
>> CONFIG_IOMMU_DEFAULT_STRICT=y
>>
>> If this happens, then make defconfig complains about both being set, 
>> and selects the 2nd, whatever that is.
> 
> On x86, Intel or AMD, this will be
> 
>      prompt "IOMMU default DMA mode"
>        depends on IOMMU_DMA
>        default IOMMU_DEFAULT_LAZY
>        default IOMMU_DEFAULT_STRICT
> 
> which will be default, LAZY or STRICT? Will it cause complains?

See Documentation/kbuild/kconfig-language.rst:

   A config option can have any number of default values. If multiple
   default values are visible, only the first defined one is active.


Robin.