Re: [Qemu-devel] modelling omap_gpmc with the hierarchical memory API

[Peter Maydell]

On 2 August 2011 16:58, Avi Kivity <address@hidden> wrote:
> On 08/02/2011 06:47 PM, Peter Maydell wrote:
>> This kind of "I want to manage the memory layout of a pile of other
>> things" seems like what the hierarchical memory API should provide,
>> but there's a bit of a difficulty here in that sysbus MMIOs aren't
>> necessarily MemoryRegions (and sysbus doesn't provide a "MemoryRegion*
>> sysbus_get_memoryregion(SysBusDevice dev, int regionnumber)"
>> anyway). How are we planning to move forward here? Will all sysbus
>> MMIOs eventually be converted to MemoryRegions?
>
> Yes.

Cool. I guess this means struct SysBusDevice's struct { ... } mmio[]
will turn into a MemoryRegion* mmio[] ?

>> Secondly, I'm not sure how to implement the gpmc size registers with
>> the memory API: memory_region_add_subregion() lets you specify the
>> offset you want to put the subregion at, but doesn't provide any way
>> to say "limit the size of the mapping to this many bytes, regardless
>> of how big the underlying device thinks it is".
>
> You can interpose you own container region:
>
>
>  system_memory
>     |
>     +--- cs_region-0
>             |
>             +--- device-connected-to-that-region
>
> cs-region-0 will clip anything under it.

OK, and when we change the size of CS0 we delete the container
region, create a new one of the new size and reconnect the
device-region to the new container? That's a bit clumsy but it
will work.

>> So maybe I'm approaching the problem wrong -- how should I be doing
>> this?
>
> I don't think those devices should be connected to the sysbus (since they
> aren't on real hardware).  Connect them to your gpmc instead.  If the
> devices are already designed for sysbus, maybe we can dual-bus them, or make
> gpmc have eight sysbuses hanging off it.

Actually I think in an ideal world omap_gpmc_attach() would
simply take a MemoryRegion* :
 void omap_gpmc_attach(DeviceState *gpmc, int cs,
                       MemoryRegion *subdevice)

(for "NOR-like" devices, with a second separate one for NAND-like
devices:
 void omap_gpmc_attach_nand(DeviceState *gpmc, int cs,
                            DeviceState *nanddevice)
...because for a NAND like device we need to do nand_setpins(),
nand_setio(), etc, but for a NOR-like device we just need to map
its memory.)

So I think we just need a sysbus_mmio_get_memoryregion()
(and convert the devices I need to attach to use memory
regions, and live with not being able to attach unconverted
devices).

Then in some future new-qemu-object-model world when devices
just directly expose their MemoryRegions it will be easy to
just pass mydevice->registers to omap_gpmc_attach().

[That is, the only reason I'm passing SysBus objects around
is that at the moment that is the only useful abstraction we
have for saying "I'm an arbitrary device object and I provide
some GPIO pins and some memory mappable regions". MemoryRegion*
allows me to pass around a memory mappable region in a more
direct way than having to pass a (SysBus*, mmio_index) tuple.]

-- PMM