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. 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
