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Subject: RE: [PATCH 02/11] PM / devfreq: Fix handling of min/max_freq == 0
Reply-To: myungjoo.ham@samsung.com
From:   MyungJoo Ham <myungjoo.ham@samsung.com>
CC:     Kyungmin Park <kyungmin.park@samsung.com>,
        Chanwoo Choi <cw00.choi@samsung.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        "linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Brian Norris <briannorris@chromium.org>,
        Douglas Anderson <dianders@chromium.org>,
        Matthias Kaehlcke <mka@chromium.org>
In-Reply-To: <20180525203043.249193-3-mka@chromium.org>
Message-ID: <20180528035953epcms1p87bf2a4ec589bd860034032e091820f69@epcms1p8>
Date:   Mon, 28 May 2018 12:59:53 +0900
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References: <20180525203043.249193-3-mka@chromium.org>
        <20180525203043.249193-1-mka@chromium.org>
        <CGME20180525203120epcas2p429d60dc21e16f0b53c58e7b1f942858f@epcms1p8>
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> Commit ab8f58ad72c4 ("PM / devfreq: Set min/max_freq when adding the
> devfreq device") initializes df->min/max_freq with the min/max OPP when
> the device is added. Later commit f1d981eaecf8 ("PM / devfreq: Use the
> available min/max frequency") adds df->scaling_min/max_freq and the
> following to the frequency adjustment code:
> 
>   max_freq = MIN(devfreq->scaling_max_freq, devfreq->max_freq);
> 
> With the current handling of min/max_freq this is incorrect:
> 
> Even though df->max_freq is now initialized to a value != 0 user space
> can still set it to 0, in this case max_freq would be 0 instead of
> df->scaling_max_freq as intended. In consequence the frequency adjustment
> is not performed:
> 
>   if (max_freq && freq > max_freq) {
> 	freq = max_freq;
> 
> To fix this set df->min/max freq to the min/max OPP in max/max_freq_store,
> when the user passes a value of 0. This also prevents df->max_freq from
> being set below the min OPP when df->min_freq is 0, and similar for
> min_freq. Since it is now guaranteed that df->min/max_freq can't be 0 the
> checks for this case can be removed.
> 
> Fixes: f1d981eaecf8 ("PM / devfreq: Use the available min/max frequency")
> Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
> ---
>  drivers/devfreq/devfreq.c | 30 ++++++++++++++++++------------
>  1 file changed, 18 insertions(+), 12 deletions(-)

Thanks a lot! Nice Catch.

Acked-by: MyungJoo Ham <myunngjoo.ham@samsung.com>

Cheers,
MyungJoo.