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    SubjectRe: [PATCH 02/11] PM / devfreq: Fix handling of min/max_freq == 0
    Hi,

    On 2018년 05월 26일 05:30, Matthias Kaehlcke wrote:
    > 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(-)
    >
    > diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
    > index 0057ef5b0a98..67da4e7b486b 100644
    > --- a/drivers/devfreq/devfreq.c
    > +++ b/drivers/devfreq/devfreq.c
    > @@ -283,11 +283,11 @@ int update_devfreq(struct devfreq *devfreq)
    > max_freq = MIN(devfreq->scaling_max_freq, devfreq->max_freq);
    > min_freq = MAX(devfreq->scaling_min_freq, devfreq->min_freq);
    >
    > - if (min_freq && freq < min_freq) {
    > + if (freq < min_freq) {
    > freq = min_freq;
    > flags &= ~DEVFREQ_FLAG_LEAST_UPPER_BOUND; /* Use GLB */
    > }
    > - if (max_freq && freq > max_freq) {
    > + if (freq > max_freq) {
    > freq = max_freq;
    > flags |= DEVFREQ_FLAG_LEAST_UPPER_BOUND; /* Use LUB */
    > }
    > @@ -1123,17 +1123,20 @@ static ssize_t min_freq_store(struct device *dev, struct device_attribute *attr,
    > struct devfreq *df = to_devfreq(dev);
    > unsigned long value;
    > int ret;
    > - unsigned long max;
    >
    > ret = sscanf(buf, "%lu", &value);
    > if (ret != 1)
    > return -EINVAL;
    >
    > mutex_lock(&df->lock);
    > - max = df->max_freq;
    > - if (value && max && value > max) {
    > - ret = -EINVAL;
    > - goto unlock;
    > +
    > + if (value) {
    > + if (value > df->max_freq) {
    > + ret = -EINVAL;
    > + goto unlock;
    > + }
    > + } else {
    > + value = df->profile->freq_table[df->profile->max_state - 1];
    > }

    If you want to prevent that df->min_freq is zero,
    you should reinitialize 'value' as following.
    Because freq_table must be in ascending order.
    value = df->profile->freq_table[0];


    >
    > df->min_freq = value;
    > @@ -1158,17 +1161,20 @@ static ssize_t max_freq_store(struct device *dev, struct device_attribute *attr,
    > struct devfreq *df = to_devfreq(dev);
    > unsigned long value;
    > int ret;
    > - unsigned long min;
    >
    > ret = sscanf(buf, "%lu", &value);
    > if (ret != 1)
    > return -EINVAL;
    >
    > mutex_lock(&df->lock);
    > - min = df->min_freq;
    > - if (value && min && value < min) {
    > - ret = -EINVAL;
    > - goto unlock;
    > +
    > + if (!value) {
    > + value = df->profile->freq_table[0];

    ditto.
    value = df->profile->freq_table[df->profile->max_state - 1];

    > + } else {
    > + if (value < df->min_freq) {
    > + ret = -EINVAL;
    > + goto unlock;
    > + }
    > }
    >
    > df->max_freq = value;
    >

    Actually, min_freq_store() and max_freq_store() are very similar.
    But, this patch changed the order of conditional statement as following:
    If there is no special reason, you better to keep the same format
    for the readability.


    min_freq_store()
    if (value) {
    ...
    } else {
    value = df->profile->freq_table[df->profile->max_state - 1];
    }


    max_freq_store()
    if (!value) {
    value = df->profile->freq_table[0];
    } else {
    ...

    --
    Best Regards,
    Chanwoo Choi
    Samsung Electronics

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