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

On 2018년 05월 30일 03:57, Matthias Kaehlcke wrote:
> On Mon, May 28, 2018 at 03:37:47PM +0900, Chanwoo Choi wrote:
>> 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];
>
> Thanks for pointing this out!
>
> The devfreq device I tested with (a Mali GPU) uses descending order
> for some reason, and I assumed that's the usual order.
>
> https://chromium.googlesource.com/chromiumos/third_party/kernel/+/chromeos-4.4/drivers/gpu/arm/midgard/backend/gpu/mali_kbase_devfreq.c#208
>
> It seems the ordering doesn't have any impact beyond this patch. If
> the order isn't mandatory for drivers that set up their own freq_table
> we should probably support both cases to be safe.

Prior to that 'freq_table' is optional. So, patch[1] initialize the 'freq_table'
by using OPP interface if 'freq_table' is NULL.
[1] commit 0ec09ac2cebe ("PM / devfreq: Set the freq_table of devfreq device")

Current devfreq recommend the ascending order for 'freq_table'.
But, as you know, it might be not enough to support them.

I agree that we should support the both cases (ascending or descending order).

Maybe, it might be not proper to access the freq_table[] directly
because we don't know the ordering style of 'freq_table'
if 'freq_table' is made by devfreq user instead of devfreq core.


>
>>> @@ -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 {
>> ...
>>
>
> Agreed, better use the same format, I'll update it in the next revision.
>
>
>


--
Best Regards,
Chanwoo Choi
Samsung Electronics

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