diff --git a/drivers/phy/socionext/phy-uniphier-usb3ss.c b/drivers/phy/socionext/phy-uniphier-usb3ss.c
index 6700645bcbe6..3b5ffc16a694 100644
--- a/drivers/phy/socionext/phy-uniphier-usb3ss.c
+++ b/drivers/phy/socionext/phy-uniphier-usb3ss.c
@@ -22,11 +22,13 @@
 #include <linux/reset.h>
 
 #define SSPHY_TESTI		0x0
-#define SSPHY_TESTO		0x4
 #define TESTI_DAT_MASK		GENMASK(13, 6)
 #define TESTI_ADR_MASK		GENMASK(5, 1)
 #define TESTI_WR_EN		BIT(0)
 
+#define SSPHY_TESTO		0x4
+#define TESTO_DAT_MASK		GENMASK(7, 0)
+
 #define PHY_F(regno, msb, lsb) { (regno), (msb), (lsb) }
 
 #define CDR_CPD_TRIM	PHY_F(7, 3, 0)	/* RxPLL charge pump current */
@@ -84,12 +86,12 @@ static void uniphier_u3ssphy_set_param(struct uniphier_u3ssphy_priv *priv,
 	val  = FIELD_PREP(TESTI_DAT_MASK, 1);
 	val |= FIELD_PREP(TESTI_ADR_MASK, p->field.reg_no);
 	uniphier_u3ssphy_testio_write(priv, val);
-	val = readl(priv->base + SSPHY_TESTO);
+	val = readl(priv->base + SSPHY_TESTO) & TESTO_DAT_MASK;
 
 	/* update value */
-	val &= ~FIELD_PREP(TESTI_DAT_MASK, field_mask);
+	val &= ~field_mask;
 	data = field_mask & (p->value << p->field.lsb);
-	val  = FIELD_PREP(TESTI_DAT_MASK, data);
+	val  = FIELD_PREP(TESTI_DAT_MASK, data | val);
 	val |= FIELD_PREP(TESTI_ADR_MASK, p->field.reg_no);
 	uniphier_u3ssphy_testio_write(priv, val);
 	uniphier_u3ssphy_testio_write(priv, val | TESTI_WR_EN);