Re: [Qemu-devel] dynamic sysbus instantiation and load_dtb implementation

[Ard Biesheuvel]

> Dear all,
>
> The goal of this mail is to summarize how dynamic sysbus device tree
> nodes were created on ARM with "machvirt dynamic sysbus device
> instantiation",
> https://lists.gnu.org/archive/html/qemu-devel/2014-09/msg01626.html
> and request some advises after commit "hw/arm/boot: load DTB as a ROM
> image", which puts into question the current implementation.
>

The reason for this change was that, before, the DTB would only be
generated once, and after a reset, the machine would go through the
kernel boot protocol as before but the DTB pointer would point to
garbage. Any idea how ppc deals with this? Do they recreate the device
tree after each reset?

-- 
Ard.

> When dynamically instantiating sysbus devices from qemu command line,
> the complete device tree cannot be built at machine init. At time we
> miss key information about those devices (base address, IRQ binding, ...)
>
> dynamic sysbus devices are "realized" after the machine init when
> parsing "-device" option line. This is at that time the information
> about the device are collected.
>
> The QEMU binding of the devices is performed in the platform_bus
> machine_init_done_notifier. Only at that time the base address of the
> device and IRQ number are chosen.
>
> The original idea was to create the dynamic sysbus device tree nodes in
> a reset callback (registered through qemu_register_reset). device tree
> was fully re-created at that time and new sysbus device nodes were added
> too. Finally archi specific load_dtb was called.
>
> On ppc/e500 this works since load_dtb uses cpu_physical_memory_write.
> it was the case on ARM too until recently but commit "hw/arm/boot: load
> DTB as a ROM image" changed the arm load_dtb implementation. It now uses
> rom_add_blob_fixed. when the reset callback is called rom_load_done()
> was called by vl.c and prevents from changing the rom content. Hence
> current callback mechanism does not work anymore.
>
> A solution I foresee to fix the issue:
> construct the device tree nodes in one machine_init_done_notifier,
> before the rom_load_done is called. I would propose the platform bus
> device (hw/core/platform-bus.c in [PATCH v3 0/7] Dynamic sysbus device
> allocation support,
> http://lists.nongnu.org/archive/html/qemu-devel/2014-09/msg04833.html)
> to register another machine_init_done_notifier whose role would be to
> initiate the dt upgrade. I would add a function to the platform bus to
> pass an opaque data that allows calling architecture specific dt
> implementation in the notifier, if needed (on ARM only).
>
> I understand reverting to previous cpu_physical_memory_write
> implementation on ARM is not the good direction.
>
> Do you have any comments about the proposed solution, any other suggestion?
>
> Thanks in advance
>
> Best Regards
>
> Eric
>
>
>
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