Re: [PATCH] hw/arm/virt: Fix PL061 node name and properties

[Peter Maydell]

On Fri, 22 May 2020 at 09:29, Geert Uytterhoeven <address@hidden> wrote:
>
> Hi Peter,
>
> On Thu, May 21, 2020 at 6:59 PM Peter Maydell <address@hidden> wrote:
> > On Tue, 19 May 2020 at 09:49, Geert Uytterhoeven
> > <address@hidden> wrote:
> > > Make the created node comply with the PL061 Device Tree bindings:
> > >   - Use generic node name "gpio" instead of "pl061",
> > >   - Add missing "#interrupt-cells" and "interrupt-controller"
> > >     properties.
> >
> > Where have these properties come from? They must be optional,
> > because in the version of the binding documentation from Linux
> > 5.0 they're not described:
> > https://elixir.bootlin.com/linux/v5.0/source/Documentation/devicetree/bindings/gpio/pl061-gpio.txt
>
> Many old DT bindings are incomplete.

Yeah, but production QEMU is out there in the world based on
the old DT binding documentation. So you can't unilaterally
make a part of the binding that wasn't documented and that QEMU
didn't emit mandatory. It might be preferable for new QEMU to
emit it, of course.

> When running the validation on a device tree passed to the guest
> (extracted from /sys/firmware/devicetree/base, converted to dts, and
>  manually fixed up the phandles), the following is reported about the
> pl061 node:
>
>     virt.dt.yaml: pl061@9030000: {'reg': [[0, 151191552, 0, 4096]],
> 'gpio-controller': True, 'phandle': [[32771]], '#gpio-cells': [[2]],
> 'clocks': [[32768]], '#interrupt-cells': [[2]], 'compatible':
> ['arm,pl061', 'arm,primecell'], 'clock-names': ['apb_pclk'],
> '$nodename': ['pl061@9030000']} is not valid under any of the given
> schemas
>     [...]
>             virt.dt.yaml: pl061@9030000: 'interrupts' is a required property
>
>     virt.dt.yaml: pl061@9030000: $nodename:0: 'pl061@9030000' does not
> match '^gpio@[0-9a-f]+$'
>     virt.dt.yaml: pl061@9030000: 'interrupt-controller' is a required property

This is just saying "the yaml says these things are mandatory".
You could equally get rid of them by marking them optional in
the yaml, right?

Also, complaining about the nodename seems like a bug in the
validation: it is not a mandatory part of the spec, just a
recommendation.

> > Since the devicetree spec says that the interrupt-controller
> > property "defines a node as an interrupt controller node"
> > and a GPIO chip isn't an interrupt controller, this seems
> > like some kind of error in the dtb binding. Maybe I'm
> > missing something...
>
> PL061 is an interrupt controller, as it can assert its interrupt output
> based on activity on GPIO input lines.

By that logic the PL011 UART is an interrupt controller, because
it can assert its interrupt output based on activity on the serial
port input lines.

A GPIO controller is not an interrupt controller inherently.
Maybe you can use it in a system design as an interrupt
controller if you want to, and in that system's dtb perhaps
it would make sense to label it as one, but the virt board's
PL061 is in no way an interrupt controller -- it's just a GPIO
controller.

> > What actually goes wrong if QEMU doesn't specify these
> > properties?
>
> It means that other devices that have their interrupt output connected
> to a PL061 GPIO input won't work, as their driver in the guest OS cannot
> find the interrupt.  Note that arm/virt.c currently doesn't instantiate
> such devices.

OK. But why would we want to run an interrupt line through the GPIO
controller when we have a perfectly good interrupt controller in
the system already?

It might be reasonable to add the properties now to avoid setting
a bear trap for ourselves in future; on the other hand if running
interrupt lines through the GPIO controller doesn't work then it
acts as a nudge to stop people adding devices that are wired
up in a weird way.

> > > diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> > > index 7dc96abf72cf2b9a..99593d7bce4d85cb 100644
> > > --- a/hw/arm/virt.c
> > > +++ b/hw/arm/virt.c
> > > @@ -818,13 +818,15 @@ static void create_gpio(const VirtMachineState *vms)
> > >                                       qdev_get_gpio_in(vms->gic, irq));
> > >
> > >      uint32_t phandle = qemu_fdt_alloc_phandle(vms->fdt);
> > > -    nodename = g_strdup_printf("/pl061@%" PRIx64, base);
> > > +    nodename = g_strdup_printf("/gpio@%" PRIx64, base);
> >
> > Does the devicetree binding really mandate what the node name is?
> > I thought that finding the right device was doe via the
> > 'compatible' string and the nodename could be whatever the
> > device tree creator wanted.
>
> Matching is indeed done based on compatible value.

OK, then we don't need to change the node name here. Lots
of the other devices on the virt board have node names that
happen to use the device name rather than being more generic.

thanks
-- PMM