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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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