Re: [PATCH 0/5] FlexRM support in VFIO platform

[Anup Patel]

On Wed, Jul 19, 2017 at 4:55 PM, Robin Murphy <robin.murphy@xxxxxxx> wrote:
> On 19/07/17 12:17, Anup Patel wrote:
>> On Wed, Jul 19, 2017 at 4:27 PM, Robin Murphy <robin.murphy@xxxxxxx> 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