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

[Damien Hedde]

On 7/31/19 7:46 AM, David Gibson wrote:
> On Tue, Jul 30, 2019 at 04:08:59PM +0200, Damien Hedde wrote:
>>
>> On 7/30/19 3:59 PM, Peter Maydell wrote:
>>> 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.
>>>
>>
>> Exactly. In the following patches, I've tried to replace existing reset
>> calls by cold or warm reset depending on whether:
>> + it is called through the main system reset -> cold
>> + it is called during normal life-time       -> warm
>>
>> But I definitely can add some docs/comments to better explain that.
> 
> Hrm, that helps, but it still seems pretty vague to me.
> 
> It's not really my call, but building the concept of warm versus cold
> resets into such a generic interface seems pretty dubios to me.  While
> it's moderately common for things to have a notion of warm versus cold
> reset it's certainly not universal.  There are many devices where
> there's no meaningful difference between the two.  There are some
> devices where there are > 2 different types of reset suitable for
> various different situations.
> 
> Is there any way this could work with it usually just presenting the
> cold reset (which is the closest to a universal concept), and any warm
> or additional resets are handled buy a different instance of the
> Resettable interface?
> 

In my current implementation, cold/warm is only a question of setting a
flag before calling the reset methods. I rely and the reset methods to
check that flag if necessary. The feature can be added/removed without
pain (if we stick with device_reset to do a cold one). So if it's
better, I can put it aside for now.

IMO handling warm reset with another interface is probably not a good
idea because it will need another load of methods.

--
Damien