Re: [PATCH 11/13] HID: playstation: add DualSense player LEDs support.

[Roderick Colenbrander <roderick@gaikai.com>]

Hi Samuel,

Thanks for your review!

On Sat, Dec 26, 2020 at 11:27 AM Samuel Čavoj <samuel@cavoj.net> wrote:
>
> Hi,
>
> I noticed that the `value` argument is not at all used in the
> player_led_set_brightness function.
>
> On 18.12.2020 22:23, Roderick Colenbrander wrote:
> > +
> > +static void dualsense_player_led_set_brightness(struct led_classdev *led, enum led_brightness value)
> > +{
> > +     struct hid_device *hdev = to_hid_device(led->dev->parent);
> > +     struct dualsense *ds = hid_get_drvdata(hdev);
> > +     uint8_t player_leds_state = 0;
> > +     unsigned long flags;
> > +     int i;
> > +
> > +     for (i = 0; i < ARRAY_SIZE(ds->player_leds); i++)
> > +             player_leds_state |= (ds->player_leds[i].brightness << i);
>
> Is it guaranteed at this point, that the led->brightness is set to the
> new value? I'm unfamiliar with the led subsystem, but skimming other
> drivers I found that they update the device based on the value of the
> `value` argument.

See comments below. Normally yes the brightness is already updated,
but as mentioned below it might still be best to change the code.

>
> > +
> > +     spin_lock_irqsave(&ds->base.lock, flags);
> > +     ds->player_leds_state = player_leds_state;
> > +     ds->update_player_leds = true;
> > +     spin_unlock_irqrestore(&ds->base.lock, flags);
> > +
> > +     schedule_work(&ds->output_worker);
> > +}
>
> Reading led-core.c, I found that led_set_brightness_{nosleep,sync} do
> set the brightness attribute, but the _nopm one does not and it is
> exported, although it is not used anywhere other than led-core.c and
> led-class.c.
>
> However, I find the usage in led_classdev_suspend and _resume
> interesting. In suspend, set_brightness_nopm is called with a value of
> 0, which should turn off the LED while retaining the value of the
> brightness attribute, which is later recalled in _resume. I assume the
> intended behaviour is the LED to turn off when suspending and return to
> its original state on resume, without overwriting the attribute.
>
> Assuming that, the "value" argument passed to dualsense_player_led_set_brightness
> can be different from led->brightness *on purpose* and should be used
> instead.
>
> I would write something along these lines:
>
> for (i = 0; i < ARRAY_SIZE(ds->player_leds); i++) {
>         if (&ds->player_leds[i] == led) {
>                 if (value == LED_OFF)
>                         player_leds_state &= ~(1 << i);
>                 else
>                         player_leds_state |= (1 << i);
>                 break;
>         }
> }
>
> This is just me hypothesizing though, could anyone clear this up for
> me? Thank you very much.

Thanks for your deep analysis. While writing the code I already found
it strange there were 2 layers of bookkeeping. As you point out this
is likely due to the power management logic.

The LEDs are created by 'ps_led_register' and it is setting:
    led->flags = LED_CORE_SUSPENDRESUME;

So those code paths would indeed be triggered provided
'CONFIG_PM_SLEEP' is set. So it is probably safest for me to change
that for-loop anyway. I had hoped to skip the 'find the LED logic' (it
just feels ugly somehow, but not a big deal). Let me make this quick
change. I will just wait a few more days on potential other feedback
from the list before submitting a 'v2'. So far nothing major, but just
these few small things :)


>
> Regards,
> Samuel

Thanks,
Roderick


-- 
Roderick Colenbrander
Senior Manager of Hardware & Systems Engineering
Sony Interactive Entertainment LLC
roderick.colenbrander@sony.com