Re: [Qemu-devel] [PATCH v3 01/33] Create Resettable QOM interface

[Damien Hedde]

On 7/31/19 12:17 PM, Christophe de Dinechin wrote:
> 
> Peter Maydell writes:
> 
>> On Tue, 30 Jul 2019 at 14:56, Cornelia Huck <address@hidden> wrote:
>>>
>>> On Tue, 30 Jul 2019 14:44:21 +0100
>>> Peter Maydell <address@hidden> wrote:
>>>
>>>> On Tue, 30 Jul 2019 at 14:42, Cornelia Huck <address@hidden> wrote:
>>>>> I'm having a hard time figuring out what a 'cold' or a 'warm' reset is
>>>>> supposed to be... can you add a definition/guideline somewhere?
>>>>
>>>> Generally "cold" reset is "power on" and "warm" is "we were already
>>>> powered-on, but somebody flipped a reset line somewhere".
>>>
>>> Ok, that makes sense... my main concern is to distinguish that in a
>>> generic way, as it is a generic interface. What about adding something
>>> like:
>>>
>>> "A 'cold' reset means that the object to be reset is initially reset; a 
>>> 'warm'
>>> reset means that the object to be reset has already been initialized."
>>>
>>> Or is that again too generic?
>>
>> I think it doesn't quite capture the idea -- an object can have already
>> been reset and then get a 'cold' reset: this is like having a powered-on
>> machine and then power-cycling it.
>>
>> The 'warm' reset is the vaguer one, because the specific behaviour
>> is somewhat device-dependent (many devices might not have any
>> difference from 'cold' reset, for those that do the exact detail
>> of what doesn't get reset on warm-reset will vary). But every
>> device should have some kind of "as if you power-cycled it" (or
>> for QEMU, "go back to the same state as if you just started QEMU on the
>> command line"). Our current "reset" method is really cold-reset.
> 
> Is there any concept of locality associated with warm reset?
> For example, you'd expect a cold reset to happen on the whole system,
> but I guess a warm reset could be restricted to a single bus.
> 
> The documentation should give examples of how warm reset could be
> triggered, and what it could do differently from cold reset.
> 

Not sure we really have some locality difference between cold/warm
resets. I think, most generally, locality of reset is on a per-device
scale with different grouping level (up to the whole system). But buses
are not always the way things are grouped.

Inside a soc (and microcontrollers in general) it follows more the clock
tree than the internal bus tree. For example on the the machine I worked
here (xilinx-zynq-a9) and, you can control by software the reset of
basically every single device and the bus too (but resetting the bus
does not reset devices on it).

On the other hand, there is some buses, like pci, which explicitly
defines some bus reset mechanism with differences between cold and warm
(some registers keep their values). (see
https://wiki.qemu.org/Features/ResetAPI)

Regarding cold reset, if a soc supports some power gating, you'll
probably have non-global cold reset in the process.

Do you mean "real world" example ?