Re: [PATCH v5 08/12] PCI: imx6: Config look up table(LUT) to support MSI ITS and IOMMU for i.MX95

[Laurentiu Tudor]

On 5/31/24 17:58, 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.

That's usually fine, except when SRIOV and/or hotplug devices (that is, 
not discoverable at bootloader time) come into play. We came up with 
this "solution" to cover these more dynamic scenarios.

https://source.denx.de/u-boot/u-boot/-/commit/2a5bbb13cc39102a68fcc31056925427ab44b591

---
Best Regards, Laurentiu

> > There's this path, which is pretty generic and does at least the
> > of_map_id() part of what you're doing in imx_pcie_add_device():
> >
> >      __driver_probe_device
> >        really_probe
> >          pci_dma_configure                       # 
> > pci_bus_type.dma_configure
> >            of_dma_configure
> >              of_dma_configure_id
> >                of_iommu_configure
> >                  of_pci_iommu_init
> >                    of_iommu_configure_dev_id
> >                      of_map_id
> >                      of_iommu_xlate
> >                        ops = iommu_ops_from_fwnode
> >                        iommu_fwspec_init
> >                        ops->of_xlate(dev, iommu_spec)
> >
> > Maybe this needs to be extended somehow with a hook to do the
> > device-specific work like updating the LUT?  Just speculating here,
> > the IOMMU folks will know how this is expected to work.
>
> Note that that particular code path has fundamental issues and much of 
> it needs to go away (I'm working on it, but it's a rich ~8-year-old pile 
> of technical debt...). IOMMU configuration needs to be happening at 
> device_add() time via the IOMMU layer's own bus notifier.
>
> 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.
>
> Thanks,
> Robin.
>
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