Re: [Qemu-devel] [PATCH qemu] fdc: Exit if ISA controller does not support DMA

[Thomas Huth <thuth@redhat.com>]

On 08.12.2017 22:29, John Snow wrote:
> 
> On 11/21/2017 09:48 PM, Alexey Kardashevskiy wrote:
>> On 07/11/17 11:58, John Snow wrote:
>>>
>>>
>>> On 10/26/2017 02:46 AM, Alexey Kardashevskiy wrote:
>>>> A "powernv" machine type defines an ISA bus but it does not add any DMA
>>>> controller to it so it is possible to hit assert(fdctrl->dma) by
>>>> adding "-machine powernv -device isa-fdc".
>>>>
>>>> This replaces assert() with an error message.
>>>>
>>>> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
>>>> ---
[...]
>>>> diff --git a/hw/block/fdc.c b/hw/block/fdc.c
>>>> index 67f78ac702..ed8b367572 100644
>>>> --- a/hw/block/fdc.c
>>>> +++ b/hw/block/fdc.c
>>>> @@ -2700,7 +2700,10 @@ static void isabus_fdc_realize(DeviceState *dev, Error **errp)
>>>>      fdctrl->dma_chann = isa->dma;
>>>>      if (fdctrl->dma_chann != -1) {
>>>>          fdctrl->dma = isa_get_dma(isa_bus_from_device(isadev), isa->dma);
>>>> -        assert(fdctrl->dma);
>>>> +        if (!fdctrl->dma) {
>>>> +            error_setg(errp, "ISA controller does not support DMA, exiting");
>>>> +            return;
>>>> +        }
>>>>      }
>>>>  
>>>>      qdev_set_legacy_instance_id(dev, isa->iobase, 2);
>>>>
>>>
>>> I've been MIA for a little while, so I'm out of the loop -- but I am not
>>> sure this is entirely the right way to fix this problem. I think it is
>>> more the case that certain boards should not be able to ask for certain
>>> types of devices, and we should prohibit e.g. powernv from being able to
>>> ask for an ISA floppy disk controller.
>>>
>>> (It doesn't seem to have an ISA DMA controller by default, but I have no
>>> idea if that means it can't EVER have one...)
>>>
>>> Papering over this by making it a soft error when we fail to execute
>>> isa_get_dma and then assuming in retrospect it's because the machine
>>> type we're on cannot have an ISA DMA controller seems a little
>>> wrong-headed. It also leaves side-effects from isa_register_portio_list
>>> and isa_init_irq, so we can't just bail here -- it's only marginally
>>> better than the assert() it's doing.
>>>
>>> That said, I am not really sure what the right thing to do is ... I
>>> suspect the "right thing" is to express the dependency that isa-fdc
>>> requires an ISA DMA controller -- and maybe that check happens here when
>>> isa_get_dma fails and we have to unwind the realize function, but we
>>> need to do it gracefully.
>>>
>>> Give me a day to think about it, but I do want to make sure this is in
>>> the next release.
>>
>> The day has passed, any news? :)
> 
> *cough* It turns out that understanding the intricacies of FDC and ISA
> is nobody's favorite thing to do.
> 
> OK, so ehabkost pointed me to this:
> 
> https://www.mail-archive.com/qemu-devel@nongnu.org/msg496460.html
> 
> Where we declare that DMA devices generally can't be created by the user
> for the inverse of the reason we're seeing here: these devices need to
> be created precisely once: not zero times, not twice, exactly once.
> 
> So we made the ISA DMA devices themselves not user-creatable, so you are
> indeed correct here that the absence of fdctrl->dma does more or less
> mean that the current configuration "doesn't support DMA." ... but maybe
> this won't always be true, and maybe some devices (TYPE_I82374?) are
> user creatable, so let's make a "softer" error message:
> 
> "No ISA DMA device present, can't create ISA FDC device."
> 
> Then, on the other end, we need to unwind realize() gracefully, maybe we
> can just shuffle up isa_get_dma() earlier so we don't have to unwind
> anything if it comes back empty.
> 
> Then I'll take the patch, because fixing this more properly I think will
> take more time or effort than I have to spend on the FDC device.

The problem still persists ... was there ever a follow-up to this patch
/ discussion?

 Thomas