linux-kernel - Re: PM regression with LED changes in next-20161109

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Date:   Fri, 11 Nov 2016 20:28:28 +0100
From:   Hans de Goede <hdegoede@...hat.com>
To:     Jacek Anaszewski <jacek.anaszewski@...il.com>,
        Pavel Machek <pavel@....cz>
Cc:     Tony Lindgren <tony@...mide.com>,
        Jacek Anaszewski <j.anaszewski@...sung.com>,
        linux-leds@...r.kernel.org, linux-omap@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Darren Hart <dvhart@...radead.org>
Subject: Re: PM regression with LED changes in next-20161109

Hi,

On 11-11-16 18:03, Jacek Anaszewski wrote:
> On 11/11/2016 01:01 PM, Pavel Machek wrote:
>> On Thu 2016-11-10 22:34:07, Jacek Anaszewski wrote:
>>> Hi,
>>>
>>> On 11/10/2016 09:29 PM, Pavel Machek wrote:
>>>> On Thu 2016-11-10 10:55:37, Tony Lindgren wrote:
>>>>> * Pavel Machek <pavel@....cz> [161110 09:29]:
>>>>>> Hi!
>>>>>>
>>>>>>>>>> Looks like commit 883d32ce3385 ("leds: core: Add support for poll()ing
>>>>>>>>>> the sysfs brightness attr for changes.") breaks runtime PM for me.
>>>>>>>>>>
>>>>>>>>>> On my omap dm3730 based test system, idle power consumption is over 70
>>>>>>>>>> times higher now with this patch! It goes from about 6mW for the core
>>>>>>>>>> system to over 440mW during idle meaning there's some busy timer now
>>>>>>>>>> active.
>>>>>>>>>>
>>>>>>>>>> Reverting this patch fixes the issue. Any ideas?
>>>>>>
>>>>>> Are you using any LED that toggles with high frequency? Like perhaps
>>>>>> LED that is lit when CPU is active?
>>>>>
>>>>> Yeah one of them seems to have cpu0 as the default trigger.
>>>>
>>>> Aha. Its quite obvious we don't want to notify sysfs each time that
>>>> one is toggled, right?
>>>>
>>>> IMO brightness should display max brightness for the trigger, as Hans
>>>> suggested, anything else is madness for trigger such as cpu activity.
>>>
>>> Are you suggesting that we should revert changes introduced
>>> by below patch?
>>>
>>> commit 29d76dfa29fe22583aefddccda0bc56aa81035dc
>>> Author: Henrique de Moraes Holschuh <hmh@....eng.br>
>>> Date:   Tue Mar 18 09:47:48 2008 +0000
>>>
>>>     leds: Add support to leds with readable status
>>>
>>>     Some led hardware allows drivers to query the led state, and this patch
>>>     adds a hook to let the led class take advantage of that information when
>>>     available.
>>>
>>>     Without this functionality, when access to the led hardware is not
>>>     exclusive (i.e. firmware or hardware might change its state behind the
>>>     kernel's back), reality goes out of sync with the led class' idea of
>>> what
>>>     the led is doing, which is annoying at best.
>>
>> Hmm. So userland can read the LED state, and it can get _some_ value
>> back, but it can not know if it is current state or not.
>>
>> I don't think that's a good interface. I see it is from 2008... is
>> someone using it? Maybe it is too late for revert.
>
> I can imagine it being used in flash LED use case. E.g. one
> could use oneshot trigger to trigger flash strobe, and then
> he could periodically read brightness file to check, for whatever
> reason, whether the flash is strobing.
>
>> But I'd certainly not extend it with poll.
>
> We could add a dedicated file e.g. hw_brightness_change for that
> (maybe someone will have a better candidate for the file name).

Why a dedicated file? Are we going to mirror brightness here
wrt r/w (show/store) behavior ? If not userspace now needs
2 open fds which is not really nice. If we are and we are
not going to use poll for something else on brightness itself
then why not just poll directly on brightness ?

Thinking more about this, I'm strongly against having to do
poll on /sys/.../bar to detect changes on /sys/.../foo that
is something which no other interface does. So my vote on this
is NACK for the having a separate file for this.

Regards,

Hans



>
> This way it would be semantically consistent to report only
> hardware originating brightness changes on it, which was the
> initial reason for adding the brightness change notification
> feature.
>
> Moreover, LED class drivers could report on this file the
> brightness level which was set by the firmware, which wouldn't
> affect current LED class device brightness setting, unless
> brightness file is read (and brightness_get op is supported).
>
>> IMO reading/polling should only be available with some triggers. It
>> does not make sense with "CPU load" trigger. It makes sense with
>> "keyboard light changeable by hardware" trigger.
>
>