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[82.37.195.13]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-499d65c326esm20415775e9.10.2026.08.25.02.15.21 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 25 Aug 2026 02:15:22 -0700 (PDT) Date: Tue, 25 Aug 2026 10:15:19 +0100 From: Daniel Thompson To: Andrew Davis Cc: "A. Sverdlin" , dri-devel@lists.freedesktop.org, linux-fbdev@vger.kernel.org, Lee Jones , Pavel Machek , Daniel Thompson , Jingoo Han , Helge Deller , linux-kernel@vger.kernel.org, linux-leds@vger.kernel.org Subject: Re: [PATCH 2/2] leds: lp8864: Register a backlight device Message-ID: References: <20260817170817.1933046-1-alexander.sverdlin@siemens.com> <20260817170817.1933046-3-alexander.sverdlin@siemens.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Mon, Aug 24, 2026 at 10:39:19AM -0500, Andrew Davis wrote: > On 8/24/26 4:33 AM, Daniel Thompson wrote: > > On Mon, Aug 17, 2026 at 07:08:15PM +0200, A. Sverdlin wrote: > > > From: Alexander Sverdlin > > > > > > The LP8864/LP8866 is a display-cluster LED backlight driver. Expose a > > > standard backlight class interface in addition to the LED class device, > > > so it can be used by display/panel stacks that expect a backlight. > > > > > > Use the new devm_led_backlight_register() helper, which spawns a > > > led-backlight device driven by our LED and tied to the I2C device > > > lifetime. This keeps the driver a plain LED driver, requires no > > > device-tree changes and remains backwards compatible with existing DTs. > > > It is a no-op when the led-backlight support is not enabled. > > > > ... but when led-backlight support is enabled it will prohibit the use > > of the LED sysfs interface. > > > > The commit message states "in addition to" so both are registered still. > The issue would be in the [1/2] patch if it disables the LED sysfs interface > as this might be unexpected behavior to not keep both interfaces active. Yes, led_bl disables the sysfs interface of any LED supplier(s). Note that this is not changed by anything in the patch series ([1/2] rearranges the code slghtlt but this has always been the case). > > Having zero DT changes is a nice property but it seems rather fragile to > > change the ABI the userspace must use to manipulate the LED based on > > CONFIG_LED_BL. > > > > If this was done in DT then it would also change the API userspace must > use in a non-flexible way (DT is also considered a fixed ABI for a board). Agreed, although it does allow an existing board not to change userspace ABI based on a Kconfig option. > > Is there no way to explictly configure which ABI should be used? > > > > I wonder if there is a good way to have the actual use of one API cause > the other to be disabled at runtime? So the first one used disables the > other, but that does sound like a race. > > Maybe a mod config to disable the LED-BL path and keep the simple LED > sysfs interface would be enough. Or maybe a sysfw entry so it could > be set per-device if needed. Mod config isn't "nice" although it might be a good enough solution for what is probably a fairly niche issue if nothing better comes up. I'd also note that a mod config would also allow the driver to error out when CONFIG_LED_BL is not set (with a hint on the logs to either enable LED_BL or set the config option) rather then the current "secret" ABI change. Daniel.