[PATCH RFC] wifi: mt76: mt792x: treat ACPI MTGS 0xff as no geo limit

[Huy Nguyen Dinh Quang]

Some OEM ACPI MTGS tables fill unused geo power/offset slots with
0xff. Windows treats that as "no cap". Linux assigned the u8 value
to s8 and min()'d it, clamping every rate to about -1 dBm
(txpower_sku tmac = 3).

Skip geo capping when pwr is 0xff, and do not apply offset when it
is 0xff.

Tested on ASUS Vivobook S 14 M5406WA (MT7922, mt7921e):
- 5 GHz: tmac 3 -> 25, TX ~40 Mbps -> 200-350 Mbps (80 MHz)
- 2.4 GHz: tmac 30 after the change (channel 6, 20 MHz)

Fixes: f965333e491e ("mt76: mt7921: introduce ACPI SAR support")
Signed-off-by: Huy Nguyen Dinh Quang <huynguyendinhquang at gmail.com>
---
 drivers/net/wireless/mediatek/mt76/mt792x_acpi_sar.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/net/wireless/mediatek/mt76/mt792x_acpi_sar.c b/drivers/net/wireless/mediatek/mt76/mt792x_acpi_sar.c
index 946dd79..fa539cd 100644
--- a/drivers/net/wireless/mediatek/mt76/mt792x_acpi_sar.c
+++ b/drivers/net/wireless/mediatek/mt76/mt792x_acpi_sar.c
@@ -280,7 +280,15 @@ mt792x_asar_get_geo_pwr(struct mt792x_phy *phy,
 		return dyn_power;
 
 	geo_power = (band_pwr + idx)->pwr;
-	dyn_power += (band_pwr + idx)->offset;
+	/*
+	 * ASUS (and some other OEM) ACPI MTGS tables fill unused geo
+	 * slots with 0xFF. Windows treats that as "no cap". Linux took
+	 * it as s8 -1 dBm and min()'d every rate down to ~3 dBm tmac.
+	 */
+	if ((u8)geo_power == 0xff)
+		return dyn_power;
+	if ((band_pwr + idx)->offset != 0xff)
+		dyn_power += (band_pwr + idx)->offset;
 
 	return min(geo_power, dyn_power);
 }
-- 
2.55.0