Re: [v7 1/5] drm/panel: add basic DP AUX backlight support

rajeevny
2021-06-23T06:15:49Z

Hi,

On 23-06-2021 00:03, Doug Anderson wrote:
> Hi,
>
> On Mon, Jun 21, 2021 at 11:38 AM Sam Ravnborg <sam@ravnborg.org> wrote:
>>
>> > > I cannot see why you need the extra check on ->enabled?
>> > > Would it be sufficient to check backlight_is_blank() only?
>> >
>> > This extra check on bl->enabled flag is added to avoid enabling/disabling
>> > backlight again if it is already enabled/disabled.
>> > Using this flag way can know the transition between backlight blank and
>> > un-blank, and decide when to enable/disable the backlight.
>>
>> My point is that this should really not be needed, as it would cover
>> up
>> for some other bug whaere we try to do something twice that is not
>> needed. But I am less certain here so if you think it is needed, keep
>> it as is.
>
> I haven't tested this myself, but I believe that it is needed. I don't
> think the backlight update_status() function is like an enable/disable
> function. I believe it can be called more than one time even while the
> backlight is disabled. For instance, you can see that
> backlight_update_status() just blindly calls through to update the
> status. That function can be called for a number of reasons. Perhaps
> Rajeev can put some printouts to confirm but I think that if the
> backlight is "blanked" for whatever reason and you write to sysfs and
> change the backlight level you'll still get called again even though
> the backlight is still "disabled".
>
Yes, sysfs write will always try to update the backlight even though the
backlight is "blanked".

The "bl->enabled" check is also required to prevent unnecessary calls to
drm_edp_backlight_enable() during every backlight level change.

To confirm this, I have added few prints in
dp_aux_backlight_update_status() function and collected the logs.
(Copying the code here to make the review easy)

static int dp_aux_backlight_update_status(struct backlight_device *bd)
{
struct dp_aux_backlight *bl = bl_get_data(bd);
u16 brightness = backlight_get_brightness(bd);
int ret = 0;

+ pr_err("%s: brightness %d, _is_blank %d, bl->enabled %d\n",
__func__,
+ brightness, backlight_is_blank(bd), bl->enabled);

if (!backlight_is_blank(bd)) {
if (!bl->enabled) {
+ pr_err("%s: enabling backlight\n", __func__);
drm_edp_backlight_enable(bl->aux, &bl->info,
brightness);
bl->enabled = true;
return 0;
}
ret = drm_edp_backlight_set_level(bl->aux, &bl->info,
brightness);
} else {
if (bl->enabled) {
+ pr_err("%s: disabling backlight\n", __func__);
drm_edp_backlight_disable(bl->aux, &bl->info);
bl->enabled = false;
}
}

return ret;
}

LOGS
====

During boot
-----------
[ 4.752188] dp_aux_backlight_update_status: brightness 102, _is_blank
0, bl->enabled 0
[ 4.760447] dp_aux_backlight_update_status: enabling backlight
[ 5.503866] dp_aux_backlight_update_status: brightness 102, _is_blank
0, bl->enabled 1
[ 6.897355] dp_aux_backlight_update_status: brightness 103, _is_blank
0, bl->enabled 1
[ 6.938617] dp_aux_backlight_update_status: brightness 104, _is_blank
0, bl->enabled 1
[ 6.980634] dp_aux_backlight_update_status: brightness 105, _is_blank
0, bl->enabled 1

Turning Panel OFF
-----------------
localhost ~ # set_power_policy --ac_screen_dim_delay=5
--ac_screen_off_delay=10
localhost ~ #

[ 106.555140] dp_aux_backlight_update_status: brightness 145, _is_blank
0, bl->enabled 1
...
...
[ 111.679407] dp_aux_backlight_update_status: brightness 7, _is_blank
0, bl->enabled 1
[ 111.700302] dp_aux_backlight_update_status: brightness 4, _is_blank
0, bl->enabled 1
[ 111.720805] dp_aux_backlight_update_status: brightness 2, _is_blank
0, bl->enabled 1
[ 111.747486] dp_aux_backlight_update_status: brightness 0, _is_blank
1, bl->enabled 1
[ 111.755580] dp_aux_backlight_update_status: disabling backlight
[ 111.792344] dp_aux_backlight_update_status: brightness 0, _is_blank
1, bl->enabled 0

Changing brightness from sysfs while panel is off
--------------------------------------------------
(it will do nothing)

localhost ~ # echo 100 >
/sys/class/backlight/dp_aux_backlight/brightness
[ 352.754963] dp_aux_backlight_update_status: brightness 0, _is_blank
1, bl->enabled 0

localhost ~ # echo 200 >
/sys/class/backlight/dp_aux_backlight/brightness
[ 364.708048] dp_aux_backlight_update_status: brightness 0, _is_blank
1, bl->enabled 0

localhost ~ # echo 0 > /sys/class/backlight/dp_aux_backlight/brightness
[ 378.850978] dp_aux_backlight_update_status: brightness 0, _is_blank
1, bl->enabled 0

Turning Panel ON
----------------
[ 553.381745] dp_aux_backlight_update_status: brightness 0, _is_blank
0, bl->enabled 0
[ 553.418133] dp_aux_backlight_update_status: enabling backlight
[ 553.426397] dp_aux_backlight_update_status: brightness 159, _is_blank
0, bl->enabled 1

====

Thanks,
Rajeev