Re: [Qemu-devel] [PATCH RFC 0/3] basic support for composing sysbus devices

[Jan Kiszka]

On 2011-06-10 15:10, Peter Maydell wrote:
> On 10 June 2011 13:51, Anthony Liguori <address@hidden> wrote:
>> On 06/10/2011 03:13 AM, Markus Armbruster wrote:
>>>
>>> Jan Kiszka<address@hidden>  writes:
>>>>
>>>> Resource management, e.g. IRQs. That will be useful for other types of
>>>> buses as well.
>>>
>>> A device should be able to say "I need to be connected to an IRQ line".
>>> Feels generic to me.
>>
>> More specifically, a device has input IRQs.  A device has no idea what
>> number the IRQ is tied to.

Generally true. Two exceptions still require to make the path explorable
/ transfer information in the reverse direction: IRQ de-coalescing and
physical device assignment. That's something a new generic API should
encapsulate so that we can stop peaking into the machine details.

>>
>> Devices may also have output IRQs.  At the qdev layer, we should be able to
>> connect an arbitrary output IRQ to an arbitrary input IRQ.
> 
> Actually, devices have input and output I/O signals (GPIOs, if you like).
> A subset of these are IRQs. We already have some APIs in QEMU which
> claim to be dealing with 'irq's but actually are just for wiring
> up generic signals; I'd rather we didn't proliferate that terminology
> confusion if possible. (And a single GPIO wire is just one kind of
> thing you might want to link between two devices, obviously. MMIO is
> another.)
> 
>> So the crux of the problem is that:
>>
>>  -device isa-serial,id=serial,irq=3
>>
>> Is very wrong.  It ought to look something more like
>>
>>  -device piix3,id=piix3 -device isa-serial,id=serial,irq=piix3.irq[3]
> 
> This makes the wiring of this signal look like a property of the
> isa-serial device, which is a bit odd, since it's just as much
> a property of the piix3. Actually it's neither, it's a property
> of the machine model, and you might actually want a syntax a bit
> more like:
> 
>  piix3 = piix3(property=value, property=value...);
>  serial = isa-serial(property=value...);
>  connect(serial.irq, piix3.irq[3]);

In fact, in the ISA case, it is a device property: The device, and only
the device decides which IRQ to use - from the bus it is attached to. So
attaching an ISA device to the bus of an ISA bridge like the PIIX3 and
selecting local IRQ 3 are the steps we can already express today.

Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux