Re: [Qemu-devel] [PATCH qemu] isa-bus: Replace assert() about DMA with error report

[John Snow <jsnow@redhat.com>]

On 10/26/2017 04:00 AM, Alexey Kardashevskiy wrote:
> Running "qemu-system-ppc64 -machine prep -device i82374" creates an ISA
> bus with two i82374 DMA controllers - one is implicit from ppc_prep_init(),
> the other one is from "-device i82374". QEMU asserts but it is not
> immediately clear why.
> 
> This adds an error message to explain the failure.
> 
> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> ---
> 
> 
> Better phase suggestions are welcome. Thanks!
> 
> 
> ---
>  hw/isa/isa-bus.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/isa/isa-bus.c b/hw/isa/isa-bus.c
> index 348e0eab9d..553707b18d 100644
> --- a/hw/isa/isa-bus.c
> +++ b/hw/isa/isa-bus.c
> @@ -107,7 +107,10 @@ void isa_connect_gpio_out(ISADevice *isadev, int gpioirq, int isairq)
>  void isa_bus_dma(ISABus *bus, IsaDma *dma8, IsaDma *dma16)
>  {
>      assert(bus && dma8 && dma16);
> -    assert(!bus->dma[0] && !bus->dma[1]);
> +    if (bus->dma[0] || bus->dma[1]) {
> +        error_setg(&error_fatal,
> +                   "DMA is already set to ISA bus, duplicated DMA controller?");
> +    }
>      bus->dma[0] = dma8;
>      bus->dma[1] = dma16;
>  }
> 

I suppose it's an improvement strictly, but really we're just naming a
runtime assertion here. We should be avoiding the assertion -- and then
how valuable is the error message?

Is this something we anticipate can never be fixed? (I.e. exclusively
the cause of asking for impossible configurations?)