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

[Alexander Graf]

On 23.10.14 12:10, Eric Auger wrote:
> 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.
> 
> 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.

The platform bus doesn't have any business in creating device trees.
Please do this in your machine file.


Alex

> 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
> 
> 
> 
> 
>