[PATCH] drm/i915/hwmon: Accept writes of value 0 to power1_max_interval

[Nilawar, Badal]

On 28-02-2023 10:13, Ashutosh Dixit wrote:
> The value shown by power1_max_interval in millisec is essentially:
> 	((1.x * power(2,y)) * 1000) >> 10
> Where x and y are read from a HW register. On ATSM, x and y are 0 on
> power-up so the value shown is 0.
> 
> Writes of 0 to power1_max_interval had previously been disallowed to avoid
> computing ilog2(0) but this resulted in the corner-case bug
> below. Therefore allow writes of 0 now but special case that write to
> x = y = 0.
> 
> Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/7754
> Signed-off-by: Ashutosh Dixit <ashutosh.dixit at intel.com>
> ---
>   drivers/gpu/drm/i915/i915_hwmon.c | 14 +++++++++-----
>   1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_hwmon.c b/drivers/gpu/drm/i915/i915_hwmon.c
> index 7c20a6f47b92e..596dd2c070106 100644
> --- a/drivers/gpu/drm/i915/i915_hwmon.c
> +++ b/drivers/gpu/drm/i915/i915_hwmon.c
> @@ -218,11 +218,15 @@ hwm_power1_max_interval_store(struct device *dev,
>   	/* val in hw units */
>   	val = DIV_ROUND_CLOSEST_ULL((u64)val << hwmon->scl_shift_time, SF_TIME);
>   	/* Convert to 1.x * power(2,y) */
> -	if (!val)
> -		return -EINVAL;
> -	y = ilog2(val);
> -	/* x = (val - (1 << y)) >> (y - 2); */
> -	x = (val - (1ul << y)) << x_w >> y;
> +	if (!val) {
> +		/* Avoid ilog2(0) */
> +		y = 0;
> +		x = 0;
> +	} else {
> +		y = ilog2(val);
> +		/* x = (val - (1 << y)) >> (y - 2); */
> +		x = (val - (1ul << y)) << x_w >> y;
> +	}
Reviewed-by: Badal Nilawar <badal.nilawar at intel.com>
>   
>   	rxy = REG_FIELD_PREP(PKG_PWR_LIM_1_TIME_X, x) | REG_FIELD_PREP(PKG_PWR_LIM_1_TIME_Y, y);
>