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 15:26:26 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     John Garry <john.garry@...wei.com>, joro@...tes.org,
        will@...nel.org, dwmw2@...radead.org, robin.murphy@....com
Cc:     baolu.lu@...ux.intel.com, 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

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?

> 
> If neither are set, then IOMMU_DEFAULT_LAZY is set if INTEL_IOMMU is 
> set, otherwise IOMMU_DEFAULT_STRICT.
> 
> Thanks,
> John

Best regards,
baolu