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

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Date:   Fri, 7 Jan 2022 16:54:01 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Daniel Lezcano <daniel.lezcano@...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

[...]

> >> +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?

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?

[...]

Kind regards
Uffe