Re: [Qemu-devel] [PATCHv2 1/4] fw_cfg: don't map the fw_cfg IO ports in fw_cfg_io_realize()

[Paolo Bonzini]

On 12/06/2017 23:21, Mark Cave-Ayland wrote:
> As indicated by Laszlo it is a QOM bug for the realize() method to actually
> map the device. Set up the IO regions with sysbus_init_mmio() and defer
> the mapping to the caller, as already done in fw_cfg_init_mem_wide().

... sort of.

The idea is that the ISA bridge (including all the legacy I/O devices,
of which fw_cfg part) does subtractive decoding, i.e. "if nobody else
wants it, I'll take it".  So that's why fw_cfg's realize() maps I/O
ports, and why the API is sysbus_add_io.

Sysbus MMIO maps a different hardware concept, where the "base" is
decoded by the SoC and forwarded to the component at that address.  This
is represented by the sysbus_init_mmio/sysbus_mmio_map pair.

Documentation for this would be welcome, but sysbus.h doesn't have many
function comments. :(

Paolo