Re: [PATCH v2] hw/block/fdc-sysbus: Always mark sysbus floppy controllers as not having DMA

[Markus Armbruster <armbru@redhat.com>]

Peter Maydell <peter.maydell@linaro.org> writes:

> The sysbus floppy controllers (devices sysbus-fdc and sun-fdtwo)
> don't support DMA.  The core floppy controller code expects this to
> be indicated by setting FDCtrl::dma_chann to -1.  This used to be
> done in the device instance_init functions sysbus_fdc_initfn() and
> sun4m_fdc_initfn(), but in commit 1430759ec3e we refactored this code
> and accidentally lost the setting of dma_chann.

Worth a

  Fixes: 1430759ec3e4cb92da224d739c914a0e8d78d786

tag?

>
> For sysbus-fdc this has no ill effects because we were redundantly
> also setting dma_chann in fdctrl_init_sysbus(), but for sun-fdtwo
> this means that guests which try to enable DMA on the floppy
> controller will cause QEMU to crash because FDCtrl::dma is NULL.
>
> Set dma_chann to -1 in the common instance init, and remove the
> redundant code in fdctrl_init_sysbus() that is also setting it.
>
> There is a six-year-old FIXME comment in the jazz board code to the
> effect that in theory it should support doing DMA via a custom DMA
> controller.  If anybody ever chooses to fix that they can do it by
> adding support for setting both FDCtrl::dma_chann and FDCtrl::dma.
> (A QOM link property 'dma-controller' on the sysbus device which can
> be set to an instance of IsaDmaClass is probably the way to go.)
>
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/958
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
> Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>