Hi Laurent, On Tue, Apr 08, 2014 at 02:50:38PM +0200, Laurent Pinchart wrote: > I agree with you, the two levels are already present, but there's still rough > edges that we need to soften. > > The ARM DMA API implementation requires someone to create the VA space mapping > by calling arm_iommu_create_mapping() and attach devices to mappings by > calling arm_iommu_attach_device(). Who is "someone" in this case? > This must only be performed for devices that use the DMA API, devices > that manage their IOMMU directly will call to the IOMMU API directly. > > One obvious possibility is to call those functions in the IOMMU bus masters > device drivers. This is pretty easy, but makes the drivers IOMMU-aware (which > I'd like to avoid) and doesn't allow multiple bus masters to share a VA space > mapping. Another possibility is to place the calls in the IOMMU drivers. This > has the advantage of removing any IOMMU knowledge from bus masters device > drivers and allowing multiple bus masters per IOMMU, but makes IOMMU > management by the DMA API unconditional. Why does that make IOMMU management by the DMA API unconditional? On x86 it works this way: The IOMMU drivers create DMA-API mappings for the devices by default. So any driver can use the DMA-API just out-of-the-box without being aware of an IOMMU. If the driver wants to deal with the IOMMU directly, it creates its own iommu-domain and attaches the device to it. The device uses the drivers domain then and not the DMA-API domain setup by the IOMMU driver. On iommu-detach the device is assigned back to its DMA-API domain. > > The way this works on x86 is that a device driver can use the DMA-API per > > default. If it wants to use the IOMMU-API it has to allocate a domain and > > add the device it handles to this domain (it can't use DMA-API anymore > > then). > > Are drivers supposed to reimplement the DMA API internally in that case? Usually not, but possible. Device drivers use the IOMMU-API when they want to control the io-addresses themselves. This is the case for VFIO for example, where a given range from a process address space is mapped into a device address space. In those cases the device driver usally does not need to implement its own address allocator. Regards, Joerg -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
