On Fri, May 31, 2024 at 03:58:49PM +0100, Robin Murphy wrote: > On 2024-05-31 12:08 am, Bjorn Helgaas wrote: > > [+cc IOMMU and pcie-apple.c folks for comment] > > > > On Tue, May 28, 2024 at 03:39:21PM -0400, Frank Li wrote: > > > For the i.MX95, configuration of a LUT is necessary to convert Bus Device > > > Function (BDF) to stream IDs, which are utilized by both IOMMU and ITS. > > > This involves examining the msi-map and smmu-map to ensure consistent > > > mapping of PCI BDF to the same stream IDs. Subsequently, LUT-related > > > registers are configured. In the absence of an msi-map, the built-in MSI > > > controller is utilized as a fallback. > > > > > > Additionally, register a PCI bus notifier to trigger imx_pcie_add_device() > > > upon the appearance of a new PCI device and when the bus is an iMX6 PCI > > > controller. This function configures the correct LUT based on Device Tree > > > Settings (DTS). > > > > This scheme is pretty similar to apple_pcie_bus_notifier(). If we > > have to do this, I wish it were *more* similar, i.e., copy the > > function names, bitmap tracking, code structure, etc. > > > > I don't really know how stream IDs work, but I assume they are used on > > most or all arm64 platforms, so I'm a little surprised that of all the > > PCI host drivers used on arm64, only pcie-apple.c and pci-imx6.c need > > this notifier. > > This is one of those things that's mostly at the mercy of the PCIe root > complex implementation. Typically the SMMU StreamID and/or GIC ITS DeviceID > is derived directly from the PCI RID, sometimes with additional high-order > bits hard-wired to disambiguate PCI segments. I believe this RID-translation > LUT is a particular feature of the the Synopsys IP - I know there's also one > on the NXP Layerscape platforms, but on those it's programmed by the > bootloader, which also generates the appropriate "msi-map" and "iommu-map" > properties to match. Ideally that's what i.MX should do as well, but hey. Maybe this RID-translation is a feature of i.MX, not of Synopsys? I see that the LUT CSR accesses use IMX95_* definitions. > If it's really necessary to do this programming from Linux, then there's > still no point in it being dynamic - the mappings cannot ever change, since > the rest of the kernel believes that what the DT said at boot time was > already a property of the hardware. It would be a lot more logical, and > likely simpler, for the driver to just read the relevant map property and > program the entire LUT to match, all in one go at controller probe time. > Rather like what's already commonly done with the parsing of "dma-ranges" to > program address-translation LUTs for inbound windows. > > Plus that would also give a chance of safely dealing with bad DTs specifying > invalid ID mappings (by refusing to probe at all). As it is, returning an > error from a child's BUS_NOTIFY_ADD_DEVICE does nothing except prevent any > further notifiers from running at that point - the device will still be > added, allowed to bind a driver, and able to start sending DMA/MSI traffic > without the controller being correctly programmed, which at best won't work > and at worst may break the whole system. Frank, could the imx LUT be programmed once at boot-time instead of at device-add time? I'm guessing maybe not because apparently there is a risk of running out of LUT entries? It sounds like the consequences of running out of LUT entries are catastrophic, e.g., memory corruption from mis-directed DMA? If that's possible, I think we need to figure out how to prevent the device from being used, not just dev_warn() about it. Bjorn
