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Sat, 17 Oct 2026 11:27:51 -0700 (PDT) X-Received: by 2002:a05:7022:3808:b0:14b:588b:c817 with SMTP id a92af1059eb24-1850c9d0734mr5109477c88.3.1792261671006; Sat, 17 Oct 2026 11:27:51 -0700 (PDT) Received: from [192.168.1.8] ([122.177.246.33]) by smtp.gmail.com with ESMTPSA id a92af1059eb24-1831b75ac0fsm15887787c88.15.2026.10.17.11.27.44 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 17 Oct 2026 11:27:50 -0700 (PDT) Message-ID: <494411c7-cf0f-4c43-9716-14fa5dc42a8a@oss.qualcomm.com> Date: Sat, 17 Oct 2026 23:57:42 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] dt-bindings: display: panel: add optional wled-supply From: Arpit Saini To: Neil Armstrong , Dmitry Baryshkov , Krzysztof Kozlowski Cc: Jessica Zhang , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Simona Vetter , Rob Herring , Krzysztof Kozlowski , Conor Dooley , dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, Krzysztof Kozlowski , devicetree@vger.kernel.org, rajeevny@qti.qualcomm.com References: <20260929-wled-supply-series-v1-0-75d0a83b4b00@oss.qualcomm.com> <20260929-wled-supply-series-v1-1-75d0a83b4b00@oss.qualcomm.com> <20261001-private-wooden-chachalaca-51aa57@quoll> <997cefa0-384c-466f-b318-077939946768@kernel.org> <1cd249d8-0a26-45e4-8263-dd4a120fc89e@oss.qualcomm.com> <54da53e6-66db-4357-a00d-da27fb936117@linaro.org> Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYxMDE3MDA3MyBTYWx0ZWRfX5mC9CdSPPqJV 6R3Sr30Rjqhfsqk/NIM9pUU4Kcsmg5of0vjSKCdO0YMbS9sa11dMBtj/kkO4mWDGconkAj7z6TX K32LjIt4Bg0/pyp4qR0OpEa/q/OB3/NVdFRIAjVvs7Gb14qEgv+ffGzxG2aIOBOv0DCvTEfthAN JrhpjlJXn3CLZ//zWHqkmhG+J2t1o7duUfqy7O3uuwoNFplfuJuqgWZCOdMuqCpxCelMFoeDvKM 1tgQD9xBsm7jeMFAsfWLKqfoxY6lNrZFMImuqZs2YJMCoL3yui9pAc0MA0vmq/aom8gRGmh2yn7 RWs+1FGPkYoeqgnQlWPbPKLFkaW2VHHkj3TZVQRRawyI3h+9dajy2KafW7sYiuxNLXTpz7kPKCS HHwlbkWVAOuJFKA2MxEUGuXb+qZViu4LHLpcr/WsmtuvWZWqm3YRPg4++jBiZHrxGKmkqWw1kYW Z788fTE4RKXjwrFgh4w== X-Authority-Analysis: v=2.4 cv=W4WtxhWk c=1 sm=1 tr=0 ts=6ad3be28 cx=c_pps a=SvEPeNj+VMjHSW//kvnxuw==:117 a=Gy7JURgNfp+0T9DH7I4t3Q==:17 a=IkcTkHD0fZMA:10 a=660iZSQnnn4A:10 a=s4-Qcg_JpJYA:10 a=VkNPw1HP01LnGYTKEx00:22 a=u7WPNUs3qKkmUXheDGA7:22 a=Um2Pa8k9VHT-vaBCBUpS:22 a=VwQbUJbxAAAA:8 a=mqesSYQOd7982J3M3jwA:9 a=3ZKOabzyN94A:10 a=QEXdDO2ut3YA:10 a=Kq8ClHjjuc5pcCNDwlU0:22 X-Proofpoint-ORIG-GUID: 7Akiv8jjXQxDjHrf2FNpMv23eS5O5lnt X-Proofpoint-GUID: 7Akiv8jjXQxDjHrf2FNpMv23eS5O5lnt X-Proofpoint-Spam-Info: AW1haW4tMjYxMDE3MDA3MyBTYWx0ZWRfXzGTUQT/JLafB QeGfifjDzJRAWpr66BwbmAJsxIb0CuF6pXWfRyFdingNFHKIEv1Qg7veZYCHhi+sWm8E+pjT3MA gOcr2R7U5hdaooT8zaLIBFaDYdNjy2s= X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1176,Hydra:6.1.134,FMLib:17.12.100.49 definitions=2026-10-17_05,2026-10-14_01,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 clxscore=1015 malwarescore=0 phishscore=0 adultscore=0 priorityscore=1501 suspectscore=0 bulkscore=0 impostorscore=0 lowpriorityscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2610080000 definitions=main-2610170073 On 10/6/2026 7:52 PM, Arpit Saini wrote: > > > On 10/2/2026 1:52 PM, Neil Armstrong wrote: >> On 10/1/26 20:09, Arpit Saini wrote: >>> >>> >>> On 10/1/2026 3:25 PM, Dmitry Baryshkov wrote: >>>> On Thu, Oct 01, 2026 at 08:57:52AM +0200, Krzysztof Kozlowski wrote: >>>>> On 01/10/2026 08:55, Krzysztof Kozlowski wrote: >>>>>> On Tue, Sep 29, 2026 at 06:42:20PM +0530, Arpit Saini wrote: >>>>>>> Some boards drive the ILI7807S panel's backlight from an external >>>>>>> WLED driver whose enable input is wired to a GPIO, typically modeled >>>>>>> as a fixed regulator (e.g. vreg_wled). >>>>>> >>>>>> You describe something else. What's fixed regulator should not matter >>>>>> here. Which pin is it in ILI7807S? >>>>>> >>>>>> It seems you just want to represent GPIO with a regulator. This is just >>>>>> confusing and typical downstream workaround. >>>>> >>>>> What's more, you basically REVERT the review YOU RECEIVED in v1. Really, >>>>> just sneak the same stuff 3 months after like the review never happened. >>>>> >>>>> NAK >>>> >>>> After discussing this offline with Krzysztof. It's not a supply (my >>>> fault), it's an LCD driver. So, the best way to handle your displaycard >>>> seems to add a gpio-backlight, reference it from the panel and then in >>>> the driver check for the backlight's max_brightness level. If it's 1, >>>> then you have to send extra DCS commands to control PWM. If it's >>>> higher, use normal backlight class controls. >>>> >>> >>> Hi Dmitry, Krzysztof >>> >>> I have a few clarifying questions regarding the proposed approach. Please let me know if I've misunderstood anything. >>>      panel_backlight: backlight { >>>        compatible = "gpio-backlight"; >>>        gpios = <&tlmm 91 GPIO_ACTIVE_HIGH>; >>>        default-on; >>>    }; >>> >>> 1) Adding gpio-backlight and check for max_brightness level if its 1, >>> >>>     If we model LCD_BKLT_EN using gpio-backlight, the backlight device effectively exposes only on/off control (max_brightness = 1), >>>     we can't support the full range of brightness i.e 0 to 16383 (0x3FFF) >>> >>> 2) Adding gpio-backlight and based upon max_brightness level of 1 , are you suggesting to register another >>>     backlight device that can actually drive DCS brightness. In that case we can actually have the MIPI DCS controlled brightness >>> >>>     If so, wouldn't that result in two backlight devices associated with the same panel: >>> >>>      gpio-backlight device for enable/disable >>>      panel backlight device for DCS brightness control >>>       Is that the expected design? >> >> It's a great question because there's a large variety of how backlight is >> implemented, and some panels can drive a PWM to an actually backlight controller >> which uses external pwm. In this case we should model the backlight IC as >> backlight driver with only 1 or 0 capability and use the DCS to program >> the PWM. >> >> So it leads exactly to your issue. So perhaps one way would be to either: >> - call into the gpio-backlight from the DCS callback, we may need to fix some locking issues >> - add way to "link" backlight devices so the backlight value can be propagated >> >> >> In any case the problem remains that both backlight devices will be exposed >> to userspace, which we don't want. So additional changes will be needed. >> >>> >>> 3) I previously tried modeling LCD_BKLT_EN using pinctrl states (panel_bl_en / panel_bl_suspend) >>>     for the enable GPIO itself. However, Dmitry suggested modeling it as a regulator instead: >>> >>>     Link : https://lore.kernel.org/all/qkhgg5x67sijiialucvzac275zhpjrtt47a4udjpyzmgvilut5@dcrslq3ai7mc/ >>> >>> 4) Modeled optional regulator wled-supply: a regulator that only gates the external backlight driver chip's power/enable, >>>     with DCS remaining the sole brightness path in this current patch , the panel-himax-hx83121a.c does exactly the same. >>>         Would this can be the preferred modeling for such panels? > > Hi Neil, > > Could you please help me understand why we cannot model the external WLED enable path similarly to panel-himax-hx83121a.c using an optional bl_supply? > > In our case, this supply would only enable or disable the external WLED driver through LCD_BKLT_EN/GPIO91, > while the panel’s existing MIPI DCS backlight would continue to control the brightness. > > This approach would also avoid exposing a second backlight device to userspace. Would this be an acceptable way to model the ILI7807S panel? > > Thanks, > Arpit Hi Dmitry, Krzysztof, Neil, Could you please clarify why we cannot model the ILI7807S backlight similarly to the approach used in panel-himax-hx83121a.c? Reference: https://lore.kernel.org/all/20260608162622.403713-3-zhengxingda@iscas.ac.cn/ Our hardware has a similar split-control arrangement: - The panel driver controls brightness through MIPI DCS commands. - An external WLED driver powers the panel backlight LEDs. - LCD_BKLT_EN / GPIO91 only enables or disables the external WLED driver. - No separate backlight device is required for brightness control. Therefore, could LCD_BKLT_EN be modeled as an optional bl-supply, with the panel driver enabling or disabling it while continuing to use DCS for brightness? This would also avoid exposing a second backlight device to userspace. If this approach is not suitable, could you please clarify what differs from the accepted panel-himax-hx83121a.c model, and advise on the preferred way to represent it ? Thanks, Arpit