On Mon, Oct 16, 2017 at 5:23 AM, Jean-Philippe Brucker <jean-philippe.brucker@xxxxxxx> wrote: > On 13/10/17 20:10, Rob Herring wrote: >> On Fri, Oct 06, 2017 at 02:31:39PM +0100, Jean-Philippe Brucker wrote: >>> On ARM systems, some platform devices behind an IOMMU may support stall >>> and PASID features. Stall is the ability to recover from page faults and >>> PASID offers multiple process address spaces to the device. Together they >>> allow to do paging with a device. Let the firmware tell us when a device >>> supports stall and PASID. >> >> Can't these be implied by the compatible string of the devices? > > I think that PASID capacity can be deduced from the compatible string but > don't know how these devices will be implemented (the only known example > being a software model for testing). In any case implementing PASID based > on the compatible string is tricky, because the IOMMU driver needs to know > PASID capacity before the device driver has had time to probe the device. > Maybe we could get away with a static table associating compatible string > to PASID capacity, but it's not very nice. > > For stall it's a property of the integration between device and IOMMU, > much like the "iommus" property, so it can't be deduced only from the > compatible string. It's crucial that the firmware validates that stalling > is safe, because we don't have any other way to discover it. Okay, fair enough. I'll let others comment. Rob
