[Intel-gfx] [PATCH 2/3] drm/i915/hwmon: Add HWMON energy support

[Dixit, Ashutosh]

On Mon, 23 May 2022 04:08:40 -0700, Badal Nilawar wrote:
>
> diff --git a/drivers/gpu/drm/i915/i915_hwmon.c b/drivers/gpu/drm/i915/i915_hwmon.c
> index b94c11f2517f..b35c4de73f30 100644
> --- a/drivers/gpu/drm/i915/i915_hwmon.c
> +++ b/drivers/gpu/drm/i915/i915_hwmon.c
> @@ -18,8 +18,10 @@
>  /*
>   * SF_* - scale factors for particular quantities according to hwmon spec.
>   * - power  - microwatts
> + * - energy - microjoules
>   */
>  #define SF_POWER	1000000
> +#define SF_ENERGY	1000000
>
>  #define FIELD_SHIFT(__mask)				    \
>	(BUILD_BUG_ON_ZERO(!__builtin_constant_p(__mask)) + \
> @@ -94,6 +96,136 @@ _field_scale_and_write(struct i915_hwmon_drvdata *ddat, i915_reg_t rgadr,
>					 bits_to_clear, bits_to_set);
>  }
>
> +/*
> + * _i915_energy1_input_sub - A custom function to obtain energy1_input.
> + * Use a custom function instead of the usual hwmon helpers in order to
> + * guarantee 64-bits of result to user-space.

We need to look into whether energy needs to be a custom interface or can
it just be the standard interface. The justification for custom interface
is that energy needs to be a u64 but the standard interface is already a
long which is at least 63 bits already. So the question is is that extra
bit needed which justifies a custom interface?

> + * Units are microjoules.
> + *
> + * The underlying hardware register is 32-bits and is subject to overflow.
> + * This function compensates for overflow of the 32-bit register by detecting
> + * wrap-around and incrementing an overflow counter.
> + * This only works if the register is sampled often enough to avoid
> + * missing an instance of overflow - achieved either by repeated
> + * queries through the API, or via a possible timer (future - TBD) that
> + * ensures values are read often enough to catch all overflows.
> + *
> + * How long before overflow?  For example, with an example scaling bit
> + * shift of 14 bits (see register *PACKAGE_POWER_SKU_UNIT) and a power draw of
> + * 1000 watts, the 32-bit counter will overflow in approximately 4.36 minutes.
> + *
> + * Examples:
> + *    1 watt:  (2^32 >> 14) /    1 W / (60 * 60 * 24) secs/day -> 3 days
> + * 1000 watts: (2^32 >> 14) / 1000 W / 60             secs/min -> 4.36 minutes

We need to "sample" the energy often enough to avoid 32 bit counter
overflow. But that can be a later patch.