On Friday, September 16, 2016 3:09:29 PM CEST Laurent Pinchart wrote: > > I wasn't thinking quite that far, though that is also a theoretical > > problem. However, the simple solution would be to have a bit in the DMA > > specifier let the driver know whether translation is needed or not. > > > > The simpler case I was thinking of is where the entire DMA engine > > either goes through an IOMMU or doesn't (depending on the integration > > into the SoC), so we'd have to find out through some DT property > > or compatible string in the DMA enginen driver. > > Don't we already get that information from the iommus DT property ? If the DMA > engine goes through an IOMMU the property will be set, otherwise it will not. It depends. A dmaengine typically at least has two DMA masters, possibly more. It's likely that some dmaengine implementations are connected to RAM through an IOMMU, but have direct access to an I/O bus for the slave FIFOs. > > > The problem is a bit broader than that, we'll also have an issue with DMA > > > engines that have different channels served by different IOMMUs. > > > > Do you mean a theoretical problem, or a chip that you already know exists? > > That's theoretical. The problem I'm facing today is a DMA engine whose > channels are served by different ports of the same IOMMU. This works in a > suboptimal way because I have to keep all the IOMMU ports enabled regardless > of whether they're used or not, as the DMA engine and IOMMU APIs don't carry > channel information. Ok Arnd
