Re: [PATCH v2 1/2] iommu/amd: Add PerfOpt IOMMU performance optimization support

[Mario Limonciello <superm1@gmail.com>]

On 9/25/26 5:39 PM, Jason Gunthorpe wrote:
> On Fri, Sep 25, 2026 at 08:11:41AM -0500, Mario Limonciello wrote:
>>
>>
>> On 9/25/26 07:28, Jason Gunthorpe wrote:
>>> On Fri, Sep 25, 2026 at 09:20:02AM +0200, Joerg Roedel wrote:
>>>> On Thu, Sep 24, 2026 at 07:50:18PM -0300, Jason Gunthorpe wrote:
>>>>> Why does the GPU driver have to request this? Why can't the iommu
>>>>> driver know that this is a special device that can use this magic fast
>>>>> path and then auto set it when identity is asked for?
>>>>
>>>> This is a layering violation either way. Either the IOMMU driver needs to poke
>>>> into device details and their relations or the device driver needs to poke into
>>>> IOMMU details.
>>>
>>> There are no device details, this is just a module option in the gpu
>>> driver.
>>
>> Well the GPU driver has knowledge whether it's a GPU inside of an APU to
>> decide if it's valid.  That information is part of the discovery table
>> setup.
> 
> Where was that in patch 2?

amdgpu_device_use_perfopt()

> 
> The right way for this to work is to quirk it through the iommu driver
> so it can choose the fast mode, and not involve the GPU driver at
> all. Like we've done for every other GPU weirdness. And AMD should be
> making this work better by providing ACPI support so the iommu driver
> can understand how it should work without inspecting PCI IDs.

There is a heuristic that Vasant added recently that we detect APU from 
the IOMMU driver.  We might be able to use that for now.

I'll take a look at what an incremental patch looks like on top of 
iommu/next that I think reworks all these paths the way you want.  I 
should be able to post something next week.