linux-kernel - Re: [PATCH V2 13/15] cpufreq: mediatek: Link CCI device to CPU

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Date:   Fri, 08 Apr 2022 13:54:36 -0700
From:   Kevin Hilman <khilman@...libre.com>
To:     Rex-BC Chen <rex-bc.chen@...iatek.com>, rafael@...nel.org,
        viresh.kumar@...aro.org, robh+dt@...nel.org, krzk+dt@...nel.org
Cc:     matthias.bgg@...il.com, jia-wei.chang@...iatek.com,
        roger.lu@...iatek.com, hsinyi@...gle.com, linux-pm@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org,
        Project_Global_Chrome_Upstream_Group@...iatek.com
Subject: Re: [PATCH V2 13/15] cpufreq: mediatek: Link CCI device to CPU

Rex-BC Chen <rex-bc.chen@...iatek.com> writes:

> From: Jia-Wei Chang <jia-wei.chang@...iatek.com>
>
> In some MediaTek SoCs, like MT8183, CPU and CCI share the same power
> supplies. Cpufreq needs to check if CCI devfreq exists and wait until
> CCI devfreq ready before scaling frequency.
>
> - Add is_ccifreq_ready() to link CCI device to CPI, and CPU will start
>   DVFS when CCI is ready.
> - Add platform data for MT8183.
>
> Signed-off-by: Jia-Wei Chang <jia-wei.chang@...iatek.com>

The checks here are not enough, and will lead to unexpected behavior.
IIUC, before doing DVFS, you're checking:

1) if the "cci" DT node is present and
2) if the driver for that device is bound

If both those conditions are not met, you don't actually fail, you just
silently do nothing in ->set_target().  As Angelo pointed out also, this
is not a good idea, and will be rather confusing to users.

The same thing would happen if the cci DT node was present, but the CCI
devfreq driver was disabled.  Silent failure would also be quite
unexpected behavior.  Similarily, if the cci DT node is not present at
all (like it is in current upstream DT), this CPUfreq driver will
silently do nothing.  Not good.

So, this patch needs to handle several scenarios:

1) CCI DT node not present

In this case, the driver should still operate normally.  With no CCI
node, or driver there's no conflict.

2) CCI DT present/enabled but not yet bound

In this case, you could return -EAGAIN as suggested by Angelo, or maybe
better, it should do a deferred probe.

3) CCI DT present, but driver disabled

This case is similar to (1), this driver should continue to work.

Kevin