Question on PCIe Device Tree bindings, Was: [PATCH] xen/arm: domain_build: Ignore device nodes with invalid addresses

[Stefano Stabellini <sstabellini@kernel.org>]

Hi Rob,

We have a question on the PCIe device tree bindings. In summary, we have
come across the Raspberry Pi 4 PCIe description below:


pcie0: pcie@7d500000 {
   compatible = "brcm,bcm2711-pcie";
   reg = <0x0 0x7d500000  0x0 0x9310>;
   device_type = "pci";
   #address-cells = <3>;
   #interrupt-cells = <1>;
   #size-cells = <2>;
   interrupts = <GIC_SPI 148 IRQ_TYPE_LEVEL_HIGH>,
                <GIC_SPI 148 IRQ_TYPE_LEVEL_HIGH>;
   interrupt-names = "pcie", "msi";
   interrupt-map-mask = <0x0 0x0 0x0 0x7>;
   interrupt-map = <0 0 0 1 &gicv2 GIC_SPI 143
                                                     IRQ_TYPE_LEVEL_HIGH>;
   msi-controller;
   msi-parent = <&pcie0>;

   ranges = <0x02000000 0x0 0xc0000000 0x6 0x00000000
             0x0 0x40000000>;
   /*
    * The wrapper around the PCIe block has a bug
    * preventing it from accessing beyond the first 3GB of
    * memory.
    */
   dma-ranges = <0x02000000 0x0 0x00000000 0x0 0x00000000
                 0x0 0xc0000000>;
   brcm,enable-ssc;

   pci@1,0 {
           #address-cells = <3>;
           #size-cells = <2>;
           ranges;

           reg = <0 0 0 0 0>;

           usb@1,0 {
                   reg = <0x10000 0 0 0 0>;
                   resets = <&reset RASPBERRYPI_FIRMWARE_RESET_ID_USB>;
           };
   };
};


Xen fails to parse it with an error because it tries to remap reg =
<0x10000 0 0 0 0> as if it was a CPU address and of course it fails.

Reading the device tree description in details, I cannot tell if Xen has
a bug: the ranges property under pci@1,0 means that pci@1,0 is treated
like a default bus (not a PCI bus), hence, the children regs are
translated using the ranges property of the parent (pcie@7d500000).

Is it possible that the device tree is missing device_type =
"pci" under pci@1,0? Or is it just implied because pci@1,0 is a child of
pcie@7d500000?

I'd like to make Xen able to parse this device tree without errors but I
am not sure what is the best way to fix it.

Thanks for any help you can provide!

Cheers,

Stefano



On Thu, 4 Feb 2021, Julien Grall wrote:
> On 04/02/2021 00:13, Stefano Stabellini wrote:
> > On Wed, 3 Feb 2021, Julien Grall wrote:
> > > On Wed, 3 Feb 2021 at 22:18, Stefano Stabellini <sstabellini@kernel.org>
> > > wrote:
> > > > > > But aside from PCIe, let's say that we know of a few nodes for which
> > > > > > "reg" needs a special treatment. I am not sure it makes sense to
> > > > > > proceed
> > > > > > with parsing those nodes without knowing how to deal with that.
> > > > > 
> > > > > I believe that most of the time the "special" treatment would be to
> > > > > ignore the
> > > > > property "regs" as it will not be an CPU memory address.
> > > > > 
> > > > > > So maybe
> > > > > > we should add those nodes to skip_matches until we know what to do
> > > > > > with
> > > > > > them. At that point, I would imagine we would introduce a special
> > > > > > handle_device function that knows what to do. In the case of PCIe,
> > > > > > something like "handle_device_pcie".
> > > > > Could you outline how "handle_device_pcie()" will differ with
> > > > > handle_node()?
> > > > > 
> > > > > In fact, the problem is not the PCIe node directly. Instead, it is the
> > > > > second
> > > > > level of nodes below it (i.e usb@...).
> > > > > 
> > > > > The current implementation of dt_number_of_address() only look at the
> > > > > bus type
> > > > > of the parent. As the parent has no bus type and "ranges" then it
> > > > > thinks this
> > > > > is something we can translate to a CPU address.
> > > > > 
> > > > > However, this is below a PCI bus so the meaning of "reg" is completely
> > > > > different. In this case, we only need to ignore "reg".
> > > > 
> > > > I see what you are saying and I agree: if we had to introduce a special
> > > > case for PCI, then  dt_number_of_address() seems to be a good place.  In
> > > > fact, we already have special PCI handling, see our
> > > > __dt_translate_address function and xen/common/device_tree.c:dt_busses.
> > > > 
> > > > Which brings the question: why is this actually failing?
> > > 
> > > I already hinted at the reason in my previous e-mail :). Let me expand
> > > a bit more.
> > > 
> > > > 
> > > > pcie0 {
> > > >       ranges = <0x02000000 0x0 0xc0000000 0x6 0x00000000 0x0
> > > > 0x40000000>;
> > > > 
> > > > Which means that PCI addresses 0xc0000000-0x100000000 become
> > > > 0x600000000-0x700000000.
> > > > 
> > > > The offending DT is:
> > > > 
> > > > &pcie0 {
> > > >           pci@1,0 {
> > > >                   #address-cells = <3>;
> > > >                   #size-cells = <2>;
> > > >                   ranges;
> > > > 
> > > >                   reg = <0 0 0 0 0>;
> > > > 
> > > >                   usb@1,0 {
> > > >                           reg = <0x10000 0 0 0 0>;
> > > >                           resets = <&reset
> > > > RASPBERRYPI_FIRMWARE_RESET_ID_USB>;
> > > >                   };
> > > >           };
> > > > };
> > > > 
> > > > 
> > > > reg = <0x10000 0 0 0 0> means that usb@1,0 is PCI device 01:00.0.
> > > > However, the rest of the regs cells are left as zero. It shouldn't be an
> > > > issue because usb@1,0 is a child of pci@1,0 but pci@1,0 is not a bus.
> > > 
> > > The property "ranges" is used to define a mapping or translation
> > > between the address space of the "bus" (here pci@1,0) and the address
> > > space of the bus node's parent (&pcie0).
> > > IOW, it means "reg" in usb@1,0 is an address on the PCI bus (i.e. BDF).
> > > 
> > > The problem is dt_number_of_address() will only look at the "bus" type
> > > of the parent using dt_match_bus(). This will return the default bus
> > > (see dt_bus_default_match()), because this is a property "ranges" in
> > > the parent node (i.e. pci@1,0). Therefore...
> > > 
> > > > So
> > > > in theory dt_number_of_address() should already return 0 for it.
> > > 
> > > ... dt_number_of_address() will return 1 even if the address is not a
> > > CPU address. So when Xen will try to translate it, it will fail.
> > > 
> > > > 
> > > > Maybe reg = <0 0 0 0 0> is the problem. In that case, we could simply
> > > > add a check to skip 0 size ranges. Just a hack to explain what I mean:
> > > 
> > > The parent of pci@1,0 is a PCI bridge (see the property type), so the
> > > CPU addresses are found not via "regs" but "assigned-addresses".
> > > 
> > > In this situation, "regs" will have a different meaning and therefore
> > > there is no promise that the size will be 0.
> > 
> > I copy/pasted the following:
> > 
> >         pci@1,0 {
> >                 #address-cells = <3>;
> >                 #size-cells = <2>;
> >                 ranges;
> > 
> >                 reg = <0 0 0 0 0>;
> > 
> >                 usb@1,0 {
> >                         reg = <0x10000 0 0 0 0>;
> >                         resets = <&reset
> >                         RASPBERRYPI_FIRMWARE_RESET_ID_USB>;
> >                 };
> >         };
> > 
> > under pcie0 in my DTS to see what happens (the node is not there in the
> > device tree for the rpi-5.9.y kernel.) It results in the expected error:
> > 
> > (XEN) Unable to retrieve address 0 for /scb/pcie@7d500000/pci@1,0/usb@1,0
> > (XEN) Device tree generation failed (-22).
> > 
> > I could verify that pci@1,0 is seen as "default" bus due to the range
> > property, thus dt_number_of_address() returns 1.
> > 
> > 
> > I can see that reg = <0 0 0 0 0> is not a problem because it is ignored
> > given that the parent is a PCI bus. assigned-addresses is the one that
> > is read.
> > 
> > 
> > But from a device tree perspective I am actually confused by the
> > presence of the "ranges" property under pci@1,0. Is that correct? It is
> > stating that addresses of children devices will be translated to the
> > address space of the parent (pcie0) using the parent translation rules.
> > I mean -- it looks like Xen is right in trying to translate reg =
> > <0x10000 0 0 0 0> using ranges = <0x02000000 0x0 0xc0000000 0x6
> > 0x00000000 0x0 0x40000000>.
> > 
> > Or maybe since pcie0 is a PCI bus all the children addresses, even
> > grand-children, are expected to be specified using "assigned-addresses"?
> > 
> > 
> > Looking at other examples [1][2] maybe the mistake is that pci@1,0 is
> > missing device_type = "pci"?  Of course, if I add that, the error
> > disappear.
> 
> I am afraid, I don't know the answer. I think it would be best to ask the
> Linux DT folks about it.
> 
> > 
> > [1] Documentation/devicetree/bindings/pci/mvebu-pci.txt
> > [2] Documentation/devicetree/bindings/pci/nvidia,tegra20-pcie.txt
> > 
> > For the sake of making Xen more resilient to possible DTSes, maybe we
> > should try to extend the dt_bus_pci_match check? See for instance the
> > change below, but we might be able to come up with better ideas.
> > 
> > 
> > diff --git a/xen/common/device_tree.c b/xen/common/device_tree.c
> > index 18825e333e..24d998f725 100644
> > --- a/xen/common/device_tree.c
> > +++ b/xen/common/device_tree.c
> > @@ -565,12 +565,21 @@ static unsigned int dt_bus_default_get_flags(const
> > __be32 *addr)
> >     static bool_t dt_bus_pci_match(const struct dt_device_node *np)
> >   {
> > +    bool ret = false;
> > +
> >       /*
> >        * "pciex" is PCI Express "vci" is for the /chaos bridge on 1st-gen
> > PCI
> >        * powermacs "ht" is hypertransport
> >        */
> > -    return !strcmp(np->type, "pci") || !strcmp(np->type, "pciex") ||
> > +    ret = !strcmp(np->type, "pci") || !strcmp(np->type, "pciex") ||
> >           !strcmp(np->type, "vci") || !strcmp(np->type, "ht");
> > +
> > +    if ( ret ) return ret;
> > +
> > +    if ( !strcmp(np->name, "pci") )
> > +        ret = dt_bus_pci_match(dt_get_parent(np));
> 
> It is probably safe to assume that a PCI device (not hostbridge) will start
> with "pci". Although, I don't much like the idea because the name is not meant
> to be stable.
> 
> AFAICT, we can only rely on "compatible" and "type".
> 
> Cheers,
> 
> -- 
> Julien Grall
>