[PATCH net] net: phy: qcom: at803x: Fix IPQ5018 short-cable DAC values

[Yongzhao Chen <yongzhao.derek@gmail.com>]

When "qcom,dac-preset-short-cable" is set, ipq5018_config_init()
programs the MDAC (MMD1 0x8100) and EDAC (debug 0x4380) fields. Both
fields occupy bits 15:8 (IPQ5018_PHY_DAC_MASK), but the value 0x10 is
passed unshifted as the set argument of phy_modify_mmd() and
at803x_debug_reg_mask(). Neither helper shifts or masks that argument,
so both fields are cleared to 0x00 instead of being set to 0x10, and
bit 4 of the low byte, which is outside the field, is set.

Use FIELD_PREP() to place the value in the field. This matches the
vendor SDK, which clears bits 15:8 and ORs in the value shifted left
by 8.

On a Redmi AX5400 board, where the IPQ5018 internal PHY connects to a
QCA8337 switch PHY without a cable, MDAC and EDAC read 0x6868 and
0x7800 before the write. With this change they read back 0x1068 and
0x1000, with the low byte preserved. Without it, the same writes would
leave 0x0078 and 0x0010.

No in-tree DTS sets this property yet, but it is documented in
qca,ar803x.yaml and used by several IPQ5018 boards in OpenWrt.

Fixes: d46502279a11 ("net: phy: qcom: at803x: Add Qualcomm IPQ5018 Internal PHY support")
Signed-off-by: Yongzhao Chen <yongzhao.derek@gmail.com>
Assisted-by: LLM
---
Testing: the register read-back above is from six boots of the Redmi
AX5400 running OpenWrt's Linux 6.18.52 kernel with this change
backported. On net, this was build-tested for arm64 (defconfig, W=1)
without new warnings; the generated code of ipq5018_config_init()
differs only in the two immediates (0x10 -> 0x1000).

 drivers/net/phy/qcom/at803x.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/net/phy/qcom/at803x.c b/drivers/net/phy/qcom/at803x.c
index 6872dbf7785..cacbadf1f48 100644
--- a/drivers/net/phy/qcom/at803x.c
+++ b/drivers/net/phy/qcom/at803x.c
@@ -1051,11 +1051,15 @@ static int ipq5018_config_init(struct phy_device *phydev)
 	if (priv->set_short_cable_dac) {
 		/* setting MDAC (Multi-level Digital-to-Analog Converter) in MMD1 */
 		phy_modify_mmd(phydev, MDIO_MMD_PMAPMD, IPQ5018_PHY_MMD1_MDAC,
-			       IPQ5018_PHY_DAC_MASK, IPQ5018_PHY_MMD1_MDAC_VAL);
+			       IPQ5018_PHY_DAC_MASK,
+			       FIELD_PREP(IPQ5018_PHY_DAC_MASK,
+					  IPQ5018_PHY_MMD1_MDAC_VAL));
 
 		/* setting EDAC (Error-detection and Correction) in debug register */
 		at803x_debug_reg_mask(phydev, IPQ5018_PHY_DEBUG_EDAC,
-				      IPQ5018_PHY_DAC_MASK, IPQ5018_PHY_DEBUG_EDAC_VAL);
+				      IPQ5018_PHY_DAC_MASK,
+				      FIELD_PREP(IPQ5018_PHY_DAC_MASK,
+						 IPQ5018_PHY_DEBUG_EDAC_VAL));
 	}
 
 	return 0;
-- 
2.43.0