linux-kernel - Re: [PATCH v1 2/3] OPP: Add function to look up required OPP's for a given OPP

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Date:   Sat, 22 Jun 2019 14:41:39 -0700
From:   Saravana Kannan <saravanak@...gle.com>
To:     cwchoi00@...il.com
Cc:     MyungJoo Ham <myungjoo.ham@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Chanwoo Choi <cw00.choi@...sung.com>,
        Viresh Kumar <vireshk@...nel.org>, Nishanth Menon <nm@...com>,
        Stephen Boyd <sboyd@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Android Kernel Team <kernel-team@...roid.com>,
        Linux PM list <linux-pm@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 2/3] OPP: Add function to look up required OPP's for a
 given OPP

On Sat, Jun 22, 2019 at 4:50 AM Chanwoo Choi <cwchoi00@...il.com> wrote:
>
> Hi,
>
> Absolutely, I like this approach. I think that it is necessary to make
> the connection
> between frequencies of devices.

Happy to hear that.

> But, I have a question on below.
>
> 2019년 6월 22일 (토) 오전 9:35, Saravana Kannan <saravanak@...gle.com>님이 작성:
> >
> > Add a function that allows looking up required OPPs given a source OPP
> > table, destination OPP table and the source OPP.
> >
> > Signed-off-by: Saravana Kannan <saravanak@...gle.com>
> > ---
> >  drivers/opp/core.c     | 54 ++++++++++++++++++++++++++++++++++++++++++
> >  include/linux/pm_opp.h | 11 +++++++++
> >  2 files changed, 65 insertions(+)
> >
> > diff --git a/drivers/opp/core.c b/drivers/opp/core.c
> > index 74c7bdc6f463..4f7870bffbf8 100644
> > --- a/drivers/opp/core.c
> > +++ b/drivers/opp/core.c
> > @@ -1830,6 +1830,60 @@ void dev_pm_opp_put_genpd_virt_dev(struct opp_table *opp_table,
> >                 dev_err(virt_dev, "Failed to find required device entry\n");
> >  }
> >
> > +/**
> > + * dev_pm_opp_xlate_opp() - Find required OPP for src_table OPP.
> > + * @src_table: OPP table which has dst_table as one of its required OPP table.
> > + * @dst_table: Required OPP table of the src_table.
> > + * @pstate: OPP of the src_table.
> > + *
> > + * This function returns the OPP (present in @dst_table) pointed out by the
> > + * "required-opps" property of the OPP (present in @src_table).
> > + *
> > + * The callers are required to call dev_pm_opp_put() for the returned OPP after
> > + * use.
> > + *
> > + * Return: destination table OPP on success, otherwise NULL on errors.
> > + */
> > +struct dev_pm_opp *dev_pm_opp_xlate_opp(struct opp_table *src_table,
> > +                                       struct opp_table *dst_table,
> > +                                       struct dev_pm_opp *src_opp)
> > +{
> > +       struct dev_pm_opp *opp, *dest_opp = NULL;
> > +       int i;
> > +
> > +       if (!src_table || !dst_table || !src_opp)
> > +               return NULL;
> > +
> > +       for (i = 0; i < src_table->required_opp_count; i++) {
> > +               if (src_table->required_opp_tables[i]->np == dst_table->np)
> > +                       break;
> > +       }
> > +
> > +       if (unlikely(i == src_table->required_opp_count)) {
> > +               pr_err("%s: Couldn't find matching OPP table (%p: %p)\n",
> > +                      __func__, src_table, dst_table);
> > +               return NULL;
> > +       }
> > +
> > +       mutex_lock(&src_table->lock);
> > +
> > +       list_for_each_entry(opp, &src_table->opp_list, node) {
> > +               if (opp == src_opp) {
> > +                       dest_opp = opp->required_opps[i];
>
> Correct me if I am wrong. This patch assume that 'i' index is same on between
> [1] and [2]. But in order to guarantee this assumption, all OPP entries
> in the same opp_table have to have the same number of 'required-opps' properties
> and keep the sequence among 'required-opps' entries.
>
> [1] src_table->required_opp_tables[i]->np
> [2] opp->required_opps[I];
>
> For example, three OPP entries in the 'parent_bus_opp'
> have the different sequence of 'required-opps' and the different
> number of 'required-opps'. Is it no problem?
>
> parent_bus_opp: opp_table {
>     compatible = "operating-points-v2";
>
>     opp2 {
>         opp-hz = /bits/ 64 <200000>;
>         required-opps = <&child_bus_a_opp2>, <&child_bus_b_opp2>,
> <&child_bus_c_opp2>;
>     };
>
>     opp1 {
>         opp-hz = /bits/ 64 <200000>;
>         // change the sequence between child_bus_b_opp2  and child_bus_c_opp2
>         required-opps = <&child_bus_a_opp2>, <&child_bus_c_opp2>,
> <&child_bus_b_opp2>
>     };
>
>     opp0 {
>         opp-hz = /bits/ 64 <200000>;
>         // missing 'child_bus_a_opp2'
>         required-opps = <&child_bus_c_opp2>, <&child_bus_b_opp2>
>     };
>
> }
>

I get your question. If I'm not mistaken the OPP framework DT parsing
code makes the assumption that the required-opps list has the phandles
in the same order for each "row" in the OPP table. It actually only
looks at the first OPP entry to figure out the list of required OPP
tables.

Technically one can write code to deal with random order of the
required-opp list, but doesn't seem like that's worth it because
there's no need to have that order all mixed up in DT. And even if
someone wants to add support for that, I don't think improving the DT
parsing to handle random order would be part of this patch series.

-Saravana

> --
> Best Regards,
> Chanwoo Choi
>
> --
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