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

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@intel.com>
Reviewed-by: Badal Nilawar <badal.nilawar@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230228044334.3630391-1-ashutosh.dixit@intel.com

diff --git a/drivers/gpu/drm/i915/i915_hwmon.c b/drivers/gpu/drm/i915/i915_hwmon.c
index 7c20a6f47b92edbea0f4386221366c981f18082f..596dd2c0701062079310475d520c5cb5c54b7bbe 100644 (file)
--- 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;
+       }
 
        rxy = REG_FIELD_PREP(PKG_PWR_LIM_1_TIME_X, x) | REG_FIELD_PREP(PKG_PWR_LIM_1_TIME_Y, y);