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Subject: Re: [PATCH v2] iommu/arm: Add module parameter to set msi iova
 address
To:     Will Deacon <will@kernel.org>,
        Vennila Megavannan <vemegava@linux.microsoft.com>
Cc:     jean-philippe@linaro.org, joro@8bytes.org,
        linux-kernel@vger.kernel.org, shameerali.kolothum.thodi@huawei.com,
        iommu@lists.linux-foundation.org, tyhicks@linux.microsoft.com,
        srinath.mannam@broadcom.com, bcm-kernel-feedback-list@broadcom.com,
        robin.murphy@arm.com, linux-arm-kernel@lists.infradead.org
References: <20200914181307.117792-1-vemegava@linux.microsoft.com>
 <20200921204545.GA3811@willie-the-truck>
From:   Auger Eric <eric.auger@redhat.com>
Message-ID: <85f7d1ae-71a9-4e95-8a30-03cc699d4794@redhat.com>
Date:   Wed, 23 Sep 2020 08:32:43 +0200
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Hi Will,

On 9/21/20 10:45 PM, Will Deacon wrote:
> On Mon, Sep 14, 2020 at 11:13:07AM -0700, Vennila Megavannan wrote:
>> From: Srinath Mannam <srinath.mannam@broadcom.com>
>>
>> Add provision to change default value of MSI IOVA base to platform's
>> suitable IOVA using module parameter. The present hardcoded MSI IOVA base
>> may not be the accessible IOVA ranges of platform.
>>
>> If any platform has the limitaion to access default MSI IOVA, then it can
>> be changed using "arm-smmu.msi_iova_base=0xa0000000" command line argument.
>>
>> Signed-off-by: Srinath Mannam <srinath.mannam@broadcom.com>
>> Co-developed-by: Vennila Megavannan <vemegava@linux.microsoft.com>
>> Signed-off-by: Vennila Megavannan <vemegava@linux.microsoft.com>
>> ---
>>  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 5 ++++-
>>  drivers/iommu/arm/arm-smmu/arm-smmu.c       | 5 ++++-
>>  2 files changed, 8 insertions(+), 2 deletions(-)
> 
> This feels pretty fragile. Wouldn't it be better to realise that there's
> a region conflict with iommu_dma_get_resv_regions() and move the MSI window
> accordingly at runtime?

Since cd2c9fcf5c66 ("iommu/dma: Move PCI window region reservation back
into dma specific path"), the PCI host bridge windows are not exposed by
iommu_dma_get_resv_regions() anymore. If I understood correctly, what is
attempted here is to avoid the collision between such PCI host bridge
window and the MSI IOVA range.

Thanks

Eric
> 
> Will
> 
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