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Kristensen" , Abhinav Kumar , Sean Paul , Kalyan Thota , Krishna Manikandan , Lee Jones , Jingoo Han , linux-fbdev@vger.kernel.org Subject: Re: [v7 1/5] drm/panel: add basic DP AUX backlight support In-Reply-To: References: <1624099230-20899-1-git-send-email-rajeevny@codeaurora.org> <1624099230-20899-2-git-send-email-rajeevny@codeaurora.org> <20210620093141.GA703072@ravnborg.org> <20210621183828.GA918146@ravnborg.org> Message-ID: X-Sender: rajeevny@codeaurora.org User-Agent: Roundcube Webmail/1.3.9 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, On 23-06-2021 00:03, Doug Anderson wrote: > Hi, > > On Mon, Jun 21, 2021 at 11:38 AM Sam Ravnborg 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