linux-kernel - RE: [PATCH RESEND V6 2/2] clk: imx: scu: add cpu frequency scaling support

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Message-ID: <155111530880.77512.16435986555733393398@swboyd.mtv.corp.google.com>
Date:   Mon, 25 Feb 2019 09:21:48 -0800
From:   Stephen Boyd <sboyd@...nel.org>
To:     "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "festevam@...il.com" <festevam@...il.com>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-clk@...r.kernel.org" <linux-clk@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "mturquette@...libre.com" <mturquette@...libre.com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "s.hauer@...gutronix.de" <s.hauer@...gutronix.de>,
        "shawnguo@...nel.org" <shawnguo@...nel.org>,
        Aisheng Dong <aisheng.dong@....com>,
        Anson Huang <anson.huang@....com>,
        Daniel Baluta <daniel.baluta@....com>
Cc:     dl-linux-imx <linux-imx@....com>
Subject: RE: [PATCH RESEND V6 2/2] clk: imx: scu: add cpu frequency scaling support

Quoting Anson Huang (2019-02-22 18:32:28)
> > > +               cluster_id = 0;
> > 
> > Do we still need to check this anymore? Why not just always use cluster_id 0?
> 
> The i.MX8QXP ONLY has 1 cluster named A35, while on i.MX8QM there will be
> 2 clusters, A53 and A72, so we need to use the resource ID to initialize the cluster_id.
> 
> > 
> > > +
> > > +       /* CPU frequency scaling can ONLY be done by ARM-Trusted-Firmware
> > */
> > > +       arm_smccc_smc(IMX_SIP_CPUFREQ, IMX_SIP_SET_CPUFREQ,
> > > +                     cluster_id, rate, 0, 0, 0, 0, &res);
> > 
> > Because not checking would make this work, vs. checking causes this code to
> > sometimes use an uninitialized value from the stack.
> 
>  89 +       if (rsrc_id == IMX_SC_R_A35)
>  90 +               init.ops = &clk_scu_cpu_ops;
>  91 +       else
>  92 +               init.ops = &clk_scu_ops;
> 
> I think it should be good. Only when plan to support cpu-freq scaling, then the
> CPU clock will be switched to use clk_scu_cpu_ops and the clutser_id initialization
> will be done according to CPU resource. For example, when we plan to support i.MX8QM
> cpu-freq scaling, we will add A53 and A72 check here and switch the clock ops to clk_scu_cpu_ops,
> also we will add the cluster_id initialization in the SMC clock set rate.
> 

Ok. So then please make the set_rate function fail if the rsrc_id
doesn't match something expected. As the code is written right now, the
compiler can't figure out that cluster_id will always be assigned, so it
complains that it may be using uninitialized data.