Without knowing the QEMU history, it is hard to relate QEMU objects with the hardware datasheet.
For example, one naively expects: * a floppy disk is plugged / unplugged on the bus Wrong! QEMU floppy disks always sit on the bus. The block drives are plugged / unplugged on the disks, and the disks magically re-adapt their proprieties to match the block drive. * a floppy controller has a fixed number of disks pluggable on the bus Wrong! QEMU floppy controllers have as much slots as the number of floppy drive provided when a machine is created. Then the ACPI table are generated and the number of slots can not be modified. So if you expect a dual slot controller being created with slot A and B, if the machine is created with a single drive attached, the controller will only have slot A created, and you will never be able to plug drive B without risking a mismatch in the ACPI tables. * a floppy controller supporting 4 disks uses 2 buses Wrong! QEMU uses a single bus to plug the 4 disks. As all these false assumptions are not obvious (we don't plug a disk, we plug a block drive into a disk, etc...), start documenting the QOM relationships with a simple ASCII schema. Signed-off-by: Philippe Mathieu-Daudé <f4...@amsat.org> --- hw/block/fdc.c | 22 ++++++++++++++++++++++ 1 file changed, 22 insertions(+) diff --git a/hw/block/fdc.c b/hw/block/fdc.c index 6944b06e4b..b109f37050 100644 --- a/hw/block/fdc.c +++ b/hw/block/fdc.c @@ -47,6 +47,28 @@ #include "qemu/module.h" #include "trace.h" +/* + * QOM relationship: + * ================= + * + * +-------------------+ + * | | + * isa/sysbus <--->| | + * | | + * irq/dma <----| fdc | + * | + * clk ---->| | +-+------+-+ +-+------+-+ + * | | | | blk | | | | blk | | + * +--------+----------+ | | | | | | | | + * | | +------+ | | +------+ | + * | | | | | + * | | floppy | | floppy | + * | +----+-----+ +----+-----+ + * | floppy-bus | | + * +------------------------v---------------v--- + * + */ + /********************************************************/ /* debug Floppy devices */ -- 2.21.3
