Re: [PATCH RFC 02/16] clk: Acquire resources for clock transitions

[Konrad Dybcio]

On 10/26/26 2:11 PM, Konrad Dybcio wrote:
> On 10/23/26 12:35 PM, Jerome Brunet wrote:
>>> Preparing a clock protects its current requirements, not the resources
>>> needed while changing its rate or parent. A transition can temporarily
>>> use both parents or access an unprepared clock, and a parent rate change
>>> can cause other clocks to be programmed too. Raising resources one clock
>>> at a time leaves those operations exposed to insufficient power.
>>>
>>> Failures make this worse: hardware may already have changed when the
>>> operation returns an error. A matching cached rate is then not proof
>>> that resource reconciliation finished. Retaining a temporary enable
>>> reference without a corresponding prepare reference also breaks a
>>> consumer's normal disable/unprepare sequence after a failed reparent.
>>>
>>> Make CCF acquire the affected tree's and parent paths' resources before
>>> notifiers or programming, and reconcile them only when the walk stops.
>>> Keep fault recovery independent of rate shortcuts and subsequent
>>> consumer reference-count changes. Require explicit recovery of an
>>> ancestor's uncertain parent selection rather than reporting success
>>> from a descendant request.
>>
>> I don't think all this is necessary to address the problem described
>> here and in the cover letter.
>>
>> IMO a new provider API is the wrong layer. The framework already has
>> the hook a provider needs: the rate change notifiers. What the core
>> should do instead is fix those notifiers, which do have a few issues.
>> You are fixing one in patch 1. Another fix, about setting the parent,
>> was sent recently:
>>
>> https://patchwork.kernel.org/project/linux-clk/patch/20261021-submit-clk-parent-abort-v1-v1-1-2936181822fc@xxxxxxxxx/
>>
>> You want to act on some changes before anything is programmed. PRE
>> notifiers are the right tool for this. The user of this change in
>> patch 7 could use them AFAICT: PRE sets a temporary vote, POST settles
>> it, ABORT drops it.
>>
>> Also, what is acquired here follows the requested clock. When an
>> ancestor switches parent, the chain of its new parent is not acquired,
>> although it feeds the whole subtree after the switch.
>>
>> With notifiers, the parent fault state is not needed either. As it is,
>> it blocks every request on or below the clock until the same switch is
>> retried.
>>
>> Could you tell me what you need from this approach that cannot be done
>> with (fixed) notifiers?
> 
> I'm a little worried about the ordering there, since some device
> drivers may also be registering listeners in parallel, so we don't
> get a clean split of what PRE means ("has power for sure " or "may
> not have power")

Do you have any thoughts on this? I'd like to proceed with this
series in one way or another.

Konrad