[PATCH v1 1/1] net: ti: am65-cpsw-nuss: Remove incorrect RGMII_ID bit functionality

[Ken Sloat]

The CPSW implementations on the AM6x platforms do not support the
selectable RGMII TX clk delay functionality via the RGMII_ID_MODE bit as
the earlier platforms did. According to various TI datasheets, reference
manuals, hardware design guides and TI forum posts from TI, this bit is
"not timed, tested, or characterized. RGMII_ID is enabled by default and
the register bit reserved."

The driver implementation today however, will incorrectly set this bit
whenever the interface mode is in RGMII_ID or RGMII_TXID.

To fix this, remove any statement setting this bit, and moreover, mask
bits 31:4 as these are all reserved.

See:
https://e2e.ti.com/support/processors-group/processors/f/processors-forum/1211306/am625-enet1_ctrl_rgmii_id-_mode
https://www.ti.com/lit/pdf/spruiv7 (Rev. B Figure 14-1717)
https://www.ti.com/lit/gpn/am625 (Rev. B Figure 7-31 Note A)
https://www.ti.com/lit/an/sprad05b/sprad05b.pdf (Rev. B Section 7.4)

Signed-off-by: Ken Sloat <ken.s at variscite.com>
---
 drivers/net/ti/am65-cpsw-nuss.c | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/drivers/net/ti/am65-cpsw-nuss.c b/drivers/net/ti/am65-cpsw-nuss.c
index 18a33c4c0e..51c432408f 100644
--- a/drivers/net/ti/am65-cpsw-nuss.c
+++ b/drivers/net/ti/am65-cpsw-nuss.c
@@ -256,7 +256,6 @@ static int am65_cpsw_gmii_sel_k3(struct am65_cpsw_priv *priv,
 {
 	struct udevice *dev = priv->dev;
 	u32 offset, reg, phandle;
-	bool rgmii_id = false;
 	fdt_addr_t gmii_sel;
 	u32 mode = 0;
 	ofnode node;
@@ -296,7 +295,6 @@ static int am65_cpsw_gmii_sel_k3(struct am65_cpsw_priv *priv,
 	case PHY_INTERFACE_MODE_RGMII_ID:
 	case PHY_INTERFACE_MODE_RGMII_TXID:
 		mode = AM65_GMII_SEL_MODE_RGMII;
-		rgmii_id = true;
 		break;
 
 	case PHY_INTERFACE_MODE_SGMII:
@@ -313,15 +311,13 @@ static int am65_cpsw_gmii_sel_k3(struct am65_cpsw_priv *priv,
 		break;
 	};
 
-	if (rgmii_id)
-		mode |= AM65_GMII_SEL_RGMII_IDMODE;
-
-	reg = mode;
+	/* bits 31:4 are reserved */
+	reg = (GENMASK(31, 4) & reg) | mode;
 	dev_dbg(dev, "gmii_sel PHY mode: %u, new gmii_sel: %08x\n",
 		phy_mode, reg);
 	writel(reg, gmii_sel);
 
-	reg = readl(gmii_sel);
+	reg = GENMASK(3, 0) & readl(gmii_sel);
 	if (reg != mode) {
 		dev_err(dev,
 			"gmii_sel PHY mode NOT SET!: requested: %08x, gmii_sel: %08x\n",
-- 
2.34.1