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Thu, 19 Mar 2026 10:50:54 +0100 From: Guy Boldon To: linux-hwmon@vger.kernel.org Cc: gb@guyboldon.com, W_Armin@gmx.de, linux@roeck-us.net, jan.claussen10@web.de Subject: Re: Weird Dell SMM bug since 6.18 Date: Thu, 19 Mar 2026 10:49:41 +0100 Message-ID: <20260319094944.239871-1-gb@guyboldon.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: References: <97b6c751-0115-4d00-b212-352f37e5914d@roeck-us.net> <02d1330f-1439-4291-bbb2-289122eedd7c@roeck-us.net> <62d86acf-6a3f-4bb2-9d81-cf47bd1461e9@roeck-us.net> Precedence: bulk X-Mailing-List: linux-hwmon@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-AuthUser: gb@guyboldon.com From: gb@guyboldon.com Hi, I'm the CoolerControl maintainer, a few notes from the userspace side. On Mon, Mar 16, 2026 at 17:55:01 -0700, Guenter Roeck wrote: > Not really. -ENODATA seems to be the correct response if the current pwm > value is not readable. Returning 0 or any other number would be misleading > and trigger other problems (such as some userspace code believing that it > can write the value back with no impact, which would be worse). For context: thinkpad_acpi has long returned 255 as a dummy value for pwmX when in auto mode (pwmX_enable=2), since it similarly cannot retrieve the real PWM value during BIOS control. This was likely motivated by fancontrol compatibility, which AFAIR requires a readable pwmX. CoolerControl reads pwmX to confirm a channel is functional and to track data values over time, hence why we need it readable. We can however adapt our handling for -ENODATA. On the write concern: several drivers I'm familiar with (e.g. nct67xx, it87, thinkpad_acpi) do not implicitly switch to manual mode on a pwmX write. Writing pwmX having no effect when pwmX_enable != 1 is expected, normal behavior from our perspective. On Tue, Mar 17, 2026 at 02:29:39 +0100, Armin Wolf wrote: > I suspect that the successful reads happen after the pwmX attribute has > been set manually using "cat". The driver will enter manual fan control > mode automatically in such a case to keep compatibility with legacy > userspace applications. That makes sense. Might be worth noting in the docs either way. As a related point: gpd_fan returns -EOPNOTSUPP rather than -ENODATA when in auto mode, and documents that behavior in the kernel docs. The inconsistency between drivers, different errors for the same condition, means userspace ends up needing per-driver handling for the same use case. Not ideal, but at least documentation helps. Thanks, Guy