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

["Philippe Mathieu-Daudé" <f4bug@amsat.org>]

On 5/22/20 11:30 AM, Peter Maydell wrote:
> On Fri, 22 May 2020 at 09:29, Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>> On Thu, May 21, 2020 at 6:59 PM Peter Maydell <peter.maydell@linaro.org> wrote:
>>> On Tue, 19 May 2020 at 09:49, Geert Uytterhoeven
>>> <geert+renesas@glider.be> 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.
>>>
[...]
>>> 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.

Yes :)

> 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?

This is sometimes done on embedded devices when all the INTC lines are 
already wired. You'd use extra lines on free peripherals. Usually the 
peripheral offer a limited GPIO mode as passthru interrupt, else you use 
nasty hacks...

> 
> 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.
[...]