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From: Anup Patel <anup.patel@broadcom.com>
Date: Wed, 19 Jul 2017 16:58:40 +0530
Message-ID: <CAALAos923XEO_oxHpqJN+XSG2MGvTc6evCf6pSg++0Qcj54vXQ@mail.gmail.com>
Subject: Re: [PATCH 0/5] FlexRM support in VFIO platform
To: Robin Murphy <robin.murphy@arm.com>
Cc: Will Deacon <will.deacon@arm.com>, Joerg Roedel <joro@8bytes.org>,
        Baptiste Reynal <b.reynal@virtualopensystems.com>,
        Alex Williamson <alex.williamson@redhat.com>,
        Scott Branden <sbranden@broadcom.com>,
        Linux Kernel <linux-kernel@vger.kernel.org>,
        Linux ARM Kernel <linux-arm-kernel@lists.infradead.org>,
        Linux IOMMU <iommu@lists.linux-foundation.org>, kvm@vger.kernel.org,
        BCM Kernel Feedback <bcm-kernel-feedback-list@broadcom.com>
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On Wed, Jul 19, 2017 at 4:55 PM, Robin Murphy <robin.murphy@arm.com> wrote:
> On 19/07/17 12:17, Anup Patel wrote:
>> On Wed, Jul 19, 2017 at 4:27 PM, Robin Murphy <robin.murphy@arm.com> wrote:
>>> On 19/07/17 10:33, Anup Patel wrote:
>>>> This patchset primarily adds Broadcom FlexRM reset module for
>>>> VFIO platform driver. We also have minor improvments in IOMMU
>>>> and VFIO driver to allow VFIO no-IOMMU mode access to FlexRM.
>>>
>>> I'm struggling to understand the IOMMU changes here - what's the
>>> FlexRM's hardware relationship with the IOMMU, and how is it different
>>> from any other device? Furthermore, if there *is* a relevant IOMMU
>>> present, why would no-IOMMU mode need to be involved at all?
>>
>> We want to have FlexRM device accessible from user-space
>> using VFIO platform with and without IOMMU.
>>
>> Currently, if IOMMU ops are available for platform bus then
>> I cannot access FlexRM device using VFIO no-IOMMU mode.
>
> So does the FlexRM hardware master through the SMMU or not? If it does,
> why do you need no-IOMMU mode? If it doesn't, then that's yet another
> reason to fix the real problem, which is the utterly broken notion of
> there being 'an IOMMU' on the platform 'bus', rather than papering over
> it in VFIO.
>

We are not trying to paper-over the issue. The ARM SMMU will
have limited number of SMRs so on a big SOC with large number
of DMA-capable devices we can run-out of SMRs if we try to
configure SMMU for all DMA-capable devices.

Regards,
Anup