Message-ID: <a82015f1-0a19-451e-9087-060a1d2c6c4d@roeck-us.net>
Date: Mon, 19 May 2025 06:17:09 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Shinji Nomoto <fj5851bi@...itsu.com>, Jean Delvare <jdelvare@...e.com>,
 Jonathan Corbet <corbet@....net>, linux-hwmon@...r.kernel.org,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] doc: hwmon: acpi_power_meter: Add information about
 enabling the power capping feature.

On 5/19/25 00:27, Shinji Nomoto wrote:
> To enable the power capping feature of the acpi_power_meter driver on
> systems other than IBM products, you must explicitly specify
> the force_cap_on module parameter.
> 
> Add information to the documentation about enabling the power capping
> feature with this driver, including the above, to improve user convenience.
> 
> Signed-off-by: Shinji Nomoto <fj5851bi@...itsu.com>
> ---
>   Documentation/hwmon/acpi_power_meter.rst | 25 +++++++++++++++++++++---
>   1 file changed, 22 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/hwmon/acpi_power_meter.rst b/Documentation/hwmon/acpi_power_meter.rst
> index 8628c1161015..334054afc498 100644
> --- a/Documentation/hwmon/acpi_power_meter.rst
> +++ b/Documentation/hwmon/acpi_power_meter.rst
> @@ -37,9 +37,16 @@ arbitrary strings that ACPI provides with the meter.  The measures/ directory
>   contains symlinks to the devices that this meter measures.
>   
>   Some computers have the ability to enforce a power cap in hardware.  If this is
> -the case, the `power[1-*]_cap` and related sysfs files will appear.  When the
> -average power consumption exceeds the cap, an ACPI event will be broadcast on
> -the netlink event socket and a poll notification will be sent to the
> +the case, the `power[1-*]_cap` and related sysfs files will appear.
> +For information on enabling the power cap feature, refer to the description
> +of the "force_on_cap" option in the "Module Parameters" chapter.
> +To use the power cap feature properly, you need to set appropriate value
> +(in microWatts) to the `power[1-*]_cap` sysfs files.
> +The value must be within the range between the minimum value at `power[1-]_cap_min`
> +and the maximum value at `power[1-]_cap_max (both in microWatts)`.
> +
> +When the average power consumption exceeds the cap, an ACPI event will be
> +broadcast on the netlink event socket and a poll notification will be sent to the
>   appropriate `power[1-*]_alarm` file to indicate that capping has begun, and the
>   hardware has taken action to reduce power consumption.  Most likely this will
>   result in reduced performance.
> @@ -52,3 +59,15 @@ follows:
>   `power[1-*]_cap` will be notified if the firmware changes the power cap.
>   `power[1-*]_interval` will be notified if the firmware changes the averaging
>   interval.
> +
> +Module Parameters
> +-----------------
> +
> +* force_cap_on: bool
> +                        Forcefully enable the power capping feature to specify
> +                        the upper limit of the system's power consumption.
> +
> +                        By default, the driver's power capping feature is only
> +                        enabled on IBM products.
> +                        Therefore, on other systems that support power capping,
> +                        you will need to use the option to enable it .

This is potentially unsafe if not really supported, which needs to be mentioned.

FWIW, if there are other systems known to support power cap in hardware, they
should be added to the driver in the dmi table. A more generic solution should
check if the _GHL and _SHL methods exist to enable the attributes. force_cap_on
should be the last resort to enable it.

Guenter

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