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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.