Re: [PATCH v9 0/8] PM / devfreq: Add dev_pm_qos support

[Leonard Crestez <leonard.crestez@nxp.com>]

On 23.10.2019 19:34, Matthias Kaehlcke wrote:
> On Wed, Oct 23, 2019 at 02:06:40PM +0000, Leonard Crestez wrote:
>> On 2019-10-02 10:25 PM, Leonard Crestez wrote:
>>> Add dev_pm_qos notifiers to devfreq core in order to support frequency
>>> limits via dev_pm_qos_add_request.
>>>
>>> Unlike the rest of devfreq the dev_pm_qos frequency is measured in Khz,
>>> this is consistent with current dev_pm_qos usage for cpufreq and
>>> allows frequencies above 2Ghz (pm_qos expresses limits as s32).
>>>
>>> Like with cpufreq the handling of min_freq/max_freq is moved to the
>>> dev_pm_qos mechanism. Constraints from userspace are no longer clamped on
>>> store, instead all values can be written and we only check against OPPs in a
>>> new devfreq_get_freq_range function. This is consistent with the design of
>>> dev_pm_qos.
>>>
>>> Notifiers from pm_qos are executed under a single global dev_pm_qos_mtx and
>>> need to take devfreq->lock. Notifier registration takes the same dev_pm_qos_mtx
>>> so in order to prevent lockdep warnings it must be done outside devfreq->lock.
>>> Current devfreq_add_device does all initialization under devfreq->lock and that
>>> needs to be relaxed.
>>>
>>> Some of first patches in the series are bugfixes and cleanups, they could be
>>> applied separately.
>>
>> Hello,
>>
>> This series was posted a few ago and all patches have been
>> reviewed/tested-by multiple people. Possible minor hangups:
>>
>> 1) Matthias found it confusing that min/max_freq in sysfs changes
>> on-the-fly. This is not a behavior change and I believe a decent
>> workaround would be to implement separate user_min/max_freq files from
>> which userspace will always read back the contraints it has written.
> 
> As you said, it isn't a behavioral change, so it shouldn't be an issue
> for this series.
> 
> Regarding the workaround I think it would be clearer to have new
> xyx_min/max_freq files for the aggregate values. min/max_freq are the
> interface userspace uses to specify the limits, it would be strange to
> use different files to read them out.
> 
> In any case the aggregate values seem to be of little practical value,
> except perhaps for monitoring, since they could be stale right after
> userspace read them out. We could start with not providing them, and add
> them if it turns out that they are actually needed.

But the min/max_freq files right now already are already aggregates and 
sysfs is considered ABI. I wouldn't be surprised if somebody has an 
userspace daemon which uses them.

My proposal is to add user_min/max_freq as a new finer-grained interface 
that you can both read and write to, no confusion here.

Writes to min/max_freq would still be supported but only for 
compatibility with older releases.

>> 2) There was an objection to removing devm from two allocs in PATCH 4. I
>> believe current solution is acceptable but a possible alternative would
>> be to split device_register into device_initialize and device_add: this
>> should allow devm sooner.
>> Link: https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fpatchwork.kernel.org%2Fpatch%2F11158385%2F%2322902151&data=02%7C01%7Cleonard.crestez%40nxp.com%7Cb89f3efc3c934030fb6108d757d6ecb2%7C686ea1d3bc2b4c6fa92cd99c5c301635%7C0%7C0%7C637074452911403311&sdata=DeOUpVjT1yZ2EWs50CFL98OoTjVMCpQDCM3qjCtKuW0%3D&reserved=0
>>
>> Let me know if you think I should implement the options above and resend.
>>
>> The bigger problem is that DEV_PM_QOS_MIN/MAX_FREQUENCY was removed from
>> pm core because only user (cpufreq) was refactored to use a new
>> interface on top of cpufreq_policy. Links to discussion:
>>       https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fpatchwork.kernel.org%2Fcover%2F11193021%2F&data=02%7C01%7Cleonard.crestez%40nxp.com%7Cb89f3efc3c934030fb6108d757d6ecb2%7C686ea1d3bc2b4c6fa92cd99c5c301635%7C0%7C0%7C637074452911408301&sdata=DxfUtaGch6MilSy5fX8AHN3%2BDIp8MrbQrHH%2B6VdRb%2FI%3D&reserved=0
>>   
>> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Flinux-pm%2FVI1PR04MB7023DF47D046AEADB4E051EBEE680%40VI1PR04MB7023.eurprd04.prod.outlook.com%2FT%2F%23u&data=02%7C01%7Cleonard.crestez%40nxp.com%7Cb89f3efc3c934030fb6108d757d6ecb2%7C686ea1d3bc2b4c6fa92cd99c5c301635%7C0%7C0%7C637074452911408301&sdata=sYQZUbzEk2DsWGQ5eQnu2m2%2Bsp%2BYBO16Abyjwf7Z1sQ%3D&reserved=0
>>
>> I believe there is still significant value in supporting min/max
>> frequency requests on a per-target-device basis. This makes much more
>> sense for devfreq that for cpufreq because the whole point of devfreq is
>> scaling arbitrary independent devices.
> 
> Agreed.
> 
> It seems Rafael would be ok with reverting the patch that removes
> DEV_PM_QOS_MIN/MAX_FREQUENCY, IIUC he just doesn't want to keep it around
> at this time because with the rest of his series there remain no in-tree
> users.