linux-kernel - Re: [PATCH v5 2/6] powercap/drivers/dtpm: Add hierarchy creation

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Date:   Tue, 11 Jan 2022 18:52:07 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     rjw@...ysocki.net, lukasz.luba@....com, robh@...nel.org,
        heiko@...ech.de, arnd@...aro.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, "Rafael J. Wysocki" <rafael@...nel.org>,
        Daniel Lezcano <daniel.lezcano@...nel.org>
Subject: Re: [PATCH v5 2/6] powercap/drivers/dtpm: Add hierarchy creation

On 11/01/2022 09:28, Ulf Hansson wrote:
> On Mon, 10 Jan 2022 at 16:55, Daniel Lezcano <daniel.lezcano@...aro.org> wrote:
>>
>> On 07/01/2022 16:54, Ulf Hansson wrote:
>>> [...]
>>>
>>>>>> +static int dtpm_for_each_child(const struct dtpm_node *hierarchy,
>>>>>> +                              const struct dtpm_node *it, struct dtpm *parent)
>>>>>> +{
>>>>>> +       struct dtpm *dtpm;
>>>>>> +       int i, ret;
>>>>>> +
>>>>>> +       for (i = 0; hierarchy[i].name; i++) {
>>>>>> +
>>>>>> +               if (hierarchy[i].parent != it)
>>>>>> +                       continue;
>>>>>> +
>>>>>> +               dtpm = dtpm_node_callback[hierarchy[i].type](&hierarchy[i], parent);
>>>>>> +               if (!dtpm || IS_ERR(dtpm))
>>>>>> +                       continue;
>>>>>> +
>>>>>> +               ret = dtpm_for_each_child(hierarchy, &hierarchy[i], dtpm);
>>>>>
>>>>> Why do you need to recursively call dtpm_for_each_child() here?
>>>>>
>>>>> Is there a restriction on how the dtpm core code manages adding
>>>>> children/parents?
>>>>
>>>> [ ... ]
>>>>
>>>> The recursive call is needed given the structure of the tree in an array
>>>> in order to connect with the parent.
>>>
>>> Right, I believe I understand what you are trying to do here, but I am
>>> not sure if this is the best approach to do this. Maybe it is.
>>>
>>> The problem is that we are also allocating memory for a dtpm and we
>>> call dtpm_register() on it in this execution path - and this memory
>>> doesn't get freed up nor unregistered, if any of the later recursive
>>> calls to dtpm_for_each_child() fails.
>>>
>>> The point is, it looks like it can get rather messy with the recursive
>>> calls to cope with the error path. Maybe it's easier to store the
>>> allocated dtpms in a list somewhere and use this to also find a
>>> reference of a parent?
>>
>> I think it is better to continue the construction with other nodes even
>> some of them failed to create, it should be a non critical issue. As an
>> analogy, if one thermal zone fails to create, the other thermal zones
>> are not removed.
> 
> Well, what if it fails because its "consumer part" is waiting for some
> resource to become available?
> 
> Maybe the devfreq driver/subsystem isn't available yet and causes
> -EPROBE_DEFER, for example. Perhaps this isn't the way the dtpm
> registration works currently, but sure it's worth considering when
> going forward, no?

It should be solved by the fact that the DTPM description is a module
and loaded after the system booted. The module loading ordering is
solved by userspace.

I agree, we could improve that but it is way too complex to be addressed
in a single series and should be part of a specific change IMO.

> In any case, papering over the error seems quite scary to me. I would
> much prefer if we instead could propagate the error code correctly to
> the caller of dtpm_create_hierarchy(), to allow it to retry if
> necessary.

It is really something we should be able to address later.

>> In addition, that should allow multiple nodes description for different
>> DT setup for the same platform. That should fix the issue pointed by Bjorn.
>>
>>> Later on, when we may decide to implement "dtpm_destroy_hierarchy()"
>>> (or whatever we would call such interface), you probably need a list
>>> of the allocated dtpms anyway, don't you think?
>>
>> No it is not necessary, the dtpms list is the dtpm tree itself and it
>> can be destroyed recursively.
> 
> I could quite figure out how that should work though, but I assume
> that is what the ->release() callback in the struct dtpm_ops is there
> to help with, in some way.
> 
> Kind regards
> Uffe
> 


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