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

[rajeevny@codeaurora.org]

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