Date:   Sun, 13 Nov 2016 14:52:30 +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 13-11-16 12:44, Jacek Anaszewski wrote:
> Hi,
>
> On 11/12/2016 10:14 PM, Hans de Goede wrote:

<snip>

>>>> So I would like to propose creating a new read-write
>>>> user_brightness file.
>>>>
>>>> The write behavior would be 100% identical to the brightness
>>>> file (in code terms it will call the same store function).
>>>>
>>>> The the read behavior otoh will be different: it will shows
>>>> the last brightness as set by the user, this would show the
>>>> read behavior we really want of brightness: show the real
>>>> brightness when not blinking / triggers are active and show
>>>> the brightness used when on when blinking / triggers are active.
>>>>
>>>> We could then add poll support on this new user_brightness
>>>> file, thus avoiding the problem with the extra cpu-load on
>>>> notifications on blinking / triggers.
>>>
>>> I agree that user_brightness allows to solve the issues you raised
>>> about inconsistent write and read brightness' semantics
>>> (which is not that painful IMHO).
>>>
>>> Reporting non-user brightness changes on user_brightness file
>>> doesn't sound reasonable though.
>>
>> The changes I'm interested in are user brightness changes they
>> are just not done through sysfs, but through a hardwired hotkey,
>> they are however very much done by the user.
>
> Ah, so this file name would be misleading especially taking into account
> the context in which "user" is used in kernel, which predominantly
> means "userspace", e.g. copy_to_user(), copy_from_user().
>
>>> Also, how would we read the
>>> brightness set by the firmware? We'd have to read brightness
>>> file, so still two files would have to be opened which is
>>> a second drawback of this approach.
>>
>> No, look carefully at the definition of the read behavior
>> I plan to put in the ABI doc:
>
> OK, "user" was what confused me. So in this case changes made
> by the firmware even if in a result of user activity
> (pressing hardware key) obviously cannot be treated similarly
> to the changes made from the userspace context.

In the end both result on the brightness of the device
changing, so any userspace process interested in monitoring
the brightness will want to know about both type of changes.

> Unless you're able to give references to the kernel code which
> contradict my judgement.

AFAIK the audio code will signal volume changes done by
hardwired buttons the same way as audio changes done
by userspace calling into the kernel. This also makes
sense because in the end, what is interesting for a
mixer app, is that the volume changed, and what the
new volume is.

>> "Reading this file will return the actual led brightness
>> when not blinking and no triggers are active; reading this
>> file will return the brightness used when the led is on
>> when blinking or triggers are active."
>
> This is unnecessarily entangled. Blinking means timer trigger
> is active.

Ok.

>> So for e.g. the backlit keyboard case reading this single
>> file will return the actual brightness of the backlight,
>> since this does not involve blinking or triggers.
>>
>> Basically the idea is that the user_brightness file
>> will have the semantics which IMHO the brightness file
>> itself should have had from the beginning, but which
>> we can't change now due to ABI reasons.
>
> And in fact introducing user_brightness file would indeed
> fix that shortcoming. However without providing notifications
> of hw brightness changes on it.

See above, I believe such a file should report any
changes in brightness, except those caused by triggers,
so it would report hw brightness changes.

Anyways if you're not interested in fixing the
shortcomings of the current read behavior on the
brightness file (I'm fine with that, I can live
with the shortcomings) I suggest that we simply go
with v2 of my poll() patch.

>>> Having no difference in this area between the two approaches
>>> I'm still in favour of the read-only file for notifying
>>> brightness changes procured by hardware.
>>
>> That brings back the needing 2 fds problem; and does
>> not solve userspace not being able to reliably read
>> the led on brightness when blinking or using triggers.
>>
>> And this also has the issue that one is doing poll() on
>> one fd to detect changes on another fd,
>
> It is not necessarily true. We can treat the polling on
> hw_brightness_change file as a means to detect brightness
> changes procured by hardware and we can read that brightness
> by executing read on this same fd. It could return -ENODATA
> if no such an event has occurred so far.

That would still require 2 fds as userspace also wants to
be able to set the keyboard backlight, but allowing read()
on the hw_brightness_change file at least fixes the weirdness
where userspace gets woken from poll() without being able to
read. So if you insist on going the hw_brightness_change file
route, then I can live with that (and upower will simply
need to open 2 fds, that is doable).

But, BUT, I would greatly prefer to just go for v4 of my
patch, which fixes the only real problem we've seen with
my patch as original merged without adding a new, somewhat
convoluted sysfs attribute.

Regards,

Hans

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