Hi, I had more thought about the reset problem and what we want to achieve (add power gating and clock support). Feel free to comment. I'll start to implement this and send a first version with a reroll of everything when it's ready.
# CONTEXT We want to model the clock distribution between devices in qemu (https://lists.gnu.org/archive/html/qemu-devel/2018-10/msg00310.html). To this end, we need to make some changes in the reset procedure because the current one does not allow to propagate things (like clocks or IOs) between devices. In the other hand, we also want to add power gating support. For that we need state support in devices. This is related to reset because powering down then up a device is like a reset. # CURRENT STATE First here is a current state of reset usage: * Buses and Devices have a single reset method taking no parameter (apart from bus/device object) * `qdev_reset_all`/`qbus_reset_all` functions call these reset methods for a bus/dev subtree + theses functions are used in ~20 places (mostly pci and scsi) * `device_reset` function only do a device local reset (does not walk the bus subtree) + device_reset is used in ~10 places * qemu_register_reset is used to register the: + `qbus_reset_all` of top-level/orphan buses (2 places) + ~100 of other reset functions (a lot of cpu related reset) * qemu_devices_reset reset everythings that has been registered with qemu_register_reset * Machines may define a reset method that overrides qemu_devices_reset global function (few machines do that) # PROPOSAL OVERVIEW I propose to add a Resetable interface and a ResetDomain object. Having an interface simplify things since there is several reset entry points (at least Buses and Devices). Device and Bus objects will implement this interface. The ResetDomain will hold several Resetable objects and implement the interface too. The ResetDomain is needed to have at least a global/main ResetDomain to place buses and devices to replace the current qemu_register_reset system which only allow to register a function. I also propose to handle the warm/cold reset distinction. This point is explained below. # RESETABLE INTERFACE This Resetable interface describes the methods corresponding to a multi- phase reset system. This is based on Peter's proposal (see https://lists.gnu.org/archive/html/qemu-devel/2018-10/msg03302.html). Here's the phases (I've tried to find adequate names): INIT PHASE: (Peter's phase 1) Reset the object internal state, put a resetting flag and do the same for the reset subtree. No side effect on other devices to guarantee that, in a reset domain, everything get initialized first. This corresponds mostly to what is currently done in the device/bus reset method. HOLD PHASE: (This phase was not in Peter's proposal) This phase allows to control a reset with a IO. When a IO control reset procedure based on the IO level (not edge), we may need to assert the reset, wait some time, and finally de-assert the reset. The consequence is that such a device can stay in a "resetting state" and may need to show this state to other devices through its outputs. For example, a clock controller will typically shutdown its clocks when it is in resetting state. RELEASE PHASE: (Peter's phase 2, modified) This phase sets outputs to state after reset. For a clock controller it starts the clocks. It also clears the "resetting" flag (this is where is differs from Peter's phase 2). A device should not react to inputs until this flag has been cleared. During this phase, outputs are propagated in the reset domain (and outside the reset domain). POST PHASE: (Peter's phase 3) This phase is kind of post-reset phase to do things that need the guarantee to be after the propagation of outputs inside the reset domain. The interface contains a method per phase: void (*reset_init)(Resetable *rst, bool cold); void (*reset_hold)(Resetable *rst); void (*reset_release)(Resetable *rst); void (*reset_post)(Resetable *rst); # WARM/COLD RESET Warm/Cold reset differences may exists in devices. We want a cold reset to handle power gating. (IMO powering off a device is identical to putting it in a cold reset state). To handle the difference between warm and cold reset, I propose to have a parameter to the init phase of Resetable interface. Other phases do not need the parameter since init phase should put warm/cold difference in the device state. # How to do a reset on a Resetable object There is 3 entry points. The first one is when we do a reset event. I call it `void resetable_trigger_reset(Resetable *rst, bool cold)`. This the one used for example at machine startup to initialize everything. this should do the init, release and postphases in a row. I don't think hold phase must be done here since release will do the opposite just after. The 2 other entry points are dedicated to io control. void resetable_assert_reset(Resetable *rst, bool cold); void resetable_deassert_reset(Resetable *rst); assert will do init and hold phases while deassert will release and post phase. Obviously to correctly support this assert/de-assert a device must have special support in order to handle migration in "resetting" state. There is no migration problem while we only use trigger because there is no point during the reset were migration could occur. # DeviceClass impact 4 new methods are added in Device: + 3 corresponding to the hold/release/post phases. + 1 to handle the cold reset init phase. The existing `reset` method is now the warm reset init phase. To handle the cold reset, a new `cold_reset` is added. The default behavior of this method is to call `reset` so that without any specific support cold and warm reset are the same. Instead of adding a cold reset method, we could add a parameter to the current reset. But there is a lot of places that would need some changes and I am really not sure we can do it automatically. # BusClass impact I don't think we need to add any new method to the bus. Since a bus has no gpio, there is no propagation so phase hold/release/term are probably meaningless here. Maybe a cold reset method is useful here ? # Impact on existing code There is not much impact. Basically the global qemu_reset should be changed to handle Resetable objects with a global ResetDomain. Also the 2 occurrences of qemu_register_reset(qbus_reset_all, ...) must be replaced to add the buses in the global ResetDomain. qdev/qbus_reset_all/device_reset behavior will be changed to trigger phase reset (instead of doing one dev/bus walk, there will be a walk per reset phase). We can add the cold/warm parameter (and modify the places were the functions are called) or choose a default (anyway until support is added to devices, both are identical). Thanks, Damien
