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

[Jason Gunthorpe <jgg@ziepe.ca>]

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.

> 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