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

[Mario Limonciello <mario.limonciello@amd.com>]

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.

> 
>> This is not perfect but to some degree unavoidable with these highly integrated
>> SOCs.
> 
> This is exactly the sort of hacky thing ACPI should describe so the
> drivers can just turn things on automatically.
> 
> Who is ever going to track down and figure out it is OK to set these
> kinds of module options in the first place? There is general negative
> sentiment in the kernel to doing things like this with modue options.
> 
> How does Windows do it? There certainly isn't a "module option" there?
> 
> Jason

This specific one I don't know; but these kinds of things usually end up 
in knobs in a UI that plumb some IPC deep into the driver stack.