[PATCH v2 4/5] drm/xe/hwmon: Decode CRI temperature registers as IEEE-754

[Karthik Poosa <karthik.poosa@intel.com>]

CRI temperature registers encode package and VRAM temperatures as
IEEE-754 floating-point values. Decode these values before exposing
them through hwmon.
 
Use integer-based IEEE-754 decoding since kernel code does not use
floating-point operations and hwmon reports integer temperatures.

Signed-off-by: Karthik Poosa <karthik.poosa@intel.com>
Assisted-by: GitHub Copilot; Model: MAI-Code-1.1-Flash
---
 drivers/gpu/drm/xe/xe_hwmon.c | 81 +++++++++++++++++++++++++++++------
 1 file changed, 67 insertions(+), 14 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_hwmon.c b/drivers/gpu/drm/xe/xe_hwmon.c
index 2a5d91fa3afa..639fdb09ab3e 100644
--- a/drivers/gpu/drm/xe/xe_hwmon.c
+++ b/drivers/gpu/drm/xe/xe_hwmon.c
@@ -1150,6 +1150,58 @@ xe_hwmon_temp_is_visible(struct xe_hwmon *hwmon, u32 attr, int channel)
 	}
 }
 
+/**
+ * xe_hwmon_temperature_decode - Decode an IEEE-754 temperature register value
+ * @reg_val: The raw register value to decode
+ *
+ * IEEE-754 single-precision format:
+ *
+ *  31 30                         23 22                         0
+ * +--+-----------------------------+----------------------------+
+ * | S|          exponent           |          fraction          |
+ * +--+-----------------------------+----------------------------+
+ *
+ * value = (-1)^S * 1.fraction * 2^(exponent - 127)
+ *
+ * Return: 0 on success or an error if @reg_val is non-finite or cannot be
+ * represented in millidegrees Celsius.
+ */
+static int xe_hwmon_temperature_decode(u32 reg_val, long *temperature)
+{
+	u32 exponent = (reg_val >> 23) & 0xff;
+	u32 mantissa = reg_val & 0x7fffff;
+	s64 value;
+	int shift;
+
+	/* Exponent 0xff encodes infinity or NaN. */
+	if (exponent == 0xff)
+		return -EINVAL;
+
+	/* Subnormal values are smaller than one millidegree Celsius. */
+	if (!exponent) {
+		*temperature = 0;
+		return 0;
+	}
+
+	value = (s64)(mantissa | BIT(23)) * MILLIDEGREE_PER_DEGREE;
+	shift = exponent - 150;
+	if (shift >= 0) {
+		if (shift >= BITS_PER_LONG || value > (LONG_MAX >> shift))
+			return -ERANGE;
+		value <<= shift;
+	} else if (shift > -64) {
+		value >>= -shift;
+	} else {
+		value = 0;
+	}
+
+	if (reg_val & BIT(31))
+		value = -value;
+
+	*temperature = value;
+	return 0;
+}
+
 static int
 xe_hwmon_temp_read(struct xe_hwmon *hwmon, u32 attr, int channel, long *val)
 {
@@ -1159,26 +1211,27 @@ xe_hwmon_temp_read(struct xe_hwmon *hwmon, u32 attr, int channel, long *val)
 	switch (attr) {
 	case hwmon_temp_input:
 		switch (channel) {
-		case CHANNEL_PKG:
-		case CHANNEL_VRAM:
-			reg_val = xe_mmio_read32(mmio, xe_hwmon_get_reg(hwmon, REG_TEMP, channel));
-
-			/* HW register value is in degrees Celsius, convert to millidegrees. */
-			*val = REG_FIELD_GET(TEMP_MASK, reg_val) * MILLIDEGREE_PER_DEGREE;
-			return 0;
 		case CHANNEL_MCTRL:
 			return get_mc_temp(hwmon, val);
 		case CHANNEL_PCIE:
 			return get_pcie_temp(hwmon, val);
+		case CHANNEL_PKG:
+		case CHANNEL_VRAM:
 		case CHANNEL_VRAM_N...CHANNEL_VRAM_N_MAX:
 			reg_val = xe_mmio_read32(mmio, xe_hwmon_get_reg(hwmon, REG_TEMP, channel));
-			/*
-			 * This temperature format is 24 bit [31:8] signed integer and 8 bit
-			 * [7:0] fraction.
-			 */
-			*val = (s32)(REG_FIELD_GET(TEMP_MASK_VRAM_N, reg_val)) *
-				(REG_FIELD_GET(TEMP_SIGN_MASK, reg_val) ? -1 : 1) *
-				 MILLIDEGREE_PER_DEGREE;
+			if (hwmon->xe->info.platform >= XE_CRESCENTISLAND) {
+				return xe_hwmon_temperature_decode((u32)reg_val, val);
+			} else if (channel >= CHANNEL_VRAM_N) {
+				/*
+				 * This temperature format is 24 bit [31:8] signed integer and 8 bit
+				 * [7:0] fraction for platforms before CRI.
+				 */
+				*val = (s32)REG_FIELD_GET(TEMP_MASK_VRAM_N, reg_val) *
+					(REG_FIELD_GET(TEMP_SIGN_MASK, reg_val) ? -1 : 1) *
+					MILLIDEGREE_PER_DEGREE;
+			} else {
+				*val = REG_FIELD_GET(TEMP_MASK, reg_val) * MILLIDEGREE_PER_DEGREE;
+			}
 			return 0;
 		default:
 			return -EOPNOTSUPP;
-- 
2.25.1