The Fast Ethernet PHYs present in the MT7628 SoCs require an
undocumented bit to be set before they can establish 100mbps links.

This commit adds the Kconfig option MEDIATEK_FE_SOC_PHY and the
corresponding driver mtk-fe-soc.c.

Signed-off-by: Joris Vaisvila <joey@tinyisr.com>
---
 drivers/net/phy/mediatek/Kconfig      | 10 +++++-
 drivers/net/phy/mediatek/Makefile     |  1 +
 drivers/net/phy/mediatek/mtk-fe-soc.c | 50 +++++++++++++++++++++++++++
 3 files changed, 60 insertions(+), 1 deletion(-)
 create mode 100644 drivers/net/phy/mediatek/mtk-fe-soc.c

diff --git a/drivers/net/phy/mediatek/Kconfig b/drivers/net/phy/mediatek/Kconfig
index bb7dc876271e..b6a51f38c358 100644
--- a/drivers/net/phy/mediatek/Kconfig
+++ b/drivers/net/phy/mediatek/Kconfig
@@ -21,8 +21,16 @@ config MEDIATEK_GE_PHY
 	  common operations with MediaTek SoC built-in Gigabit
 	  Ethernet PHYs.
 
+config MEDIATEK_FE_SOC_PHY
+	tristate "MediaTek SoC Fast Ethernet PHYs"
+	help
+	  Support for MediaTek MT7628 built-in Fast Ethernet PHYs.
+	  This driver only sets an initialization bit required for the PHY
+	  to establish 100 Mbps links. All other PHY operations are handled
+	  by the kernel's generic PHY code.
+
 config MEDIATEK_GE_SOC_PHY
-	tristate "MediaTek SoC Ethernet PHYs"
+	tristate "MediaTek SoC Gigabit Ethernet PHYs"
 	depends on ARM64 || COMPILE_TEST
 	depends on ARCH_AIROHA || (ARCH_MEDIATEK && NVMEM_MTK_EFUSE) || \
 		   COMPILE_TEST
diff --git a/drivers/net/phy/mediatek/Makefile b/drivers/net/phy/mediatek/Makefile
index ac57ecc799fc..6f9cacf7f906 100644
--- a/drivers/net/phy/mediatek/Makefile
+++ b/drivers/net/phy/mediatek/Makefile
@@ -1,5 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0
 obj-$(CONFIG_MEDIATEK_2P5GE_PHY)	+= mtk-2p5ge.o
+obj-$(CONFIG_MEDIATEK_FE_SOC_PHY)	+= mtk-fe-soc.o
 obj-$(CONFIG_MEDIATEK_GE_PHY)		+= mtk-ge.o
 obj-$(CONFIG_MEDIATEK_GE_SOC_PHY)	+= mtk-ge-soc.o
 obj-$(CONFIG_MTK_NET_PHYLIB)		+= mtk-phy-lib.o
diff --git a/drivers/net/phy/mediatek/mtk-fe-soc.c b/drivers/net/phy/mediatek/mtk-fe-soc.c
new file mode 100644
index 000000000000..317944411fbe
--- /dev/null
+++ b/drivers/net/phy/mediatek/mtk-fe-soc.c
@@ -0,0 +1,50 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Driver for MT7628 Embedded Switch internal Fast Ethernet PHYs
+ */
+#include <linux/module.h>
+#include <linux/phy.h>
+
+#define MTK_FPHY_ID_MT7628	0x03a29410
+#define MTK_EXT_PAGE_ACCESS	0x1f
+
+static int mt7628_phy_read_page(struct phy_device *phydev)
+{
+	return __phy_read(phydev, MTK_EXT_PAGE_ACCESS);
+}
+
+static int mt7628_phy_write_page(struct phy_device *phydev, int page)
+{
+	return __phy_write(phydev, MTK_EXT_PAGE_ACCESS, page);
+}
+
+static int mt7628_phy_config_init(struct phy_device *phydev)
+{
+	/*
+	 * This undocumented bit is required for the PHYs to be able to
+	 * establish 100mbps links.
+	 */
+	return phy_write_paged(phydev, 0x8000, 30, BIT(13));
+}
+
+static struct phy_driver mtk_soc_fe_phy_driver[] = {
+	{
+		PHY_ID_MATCH_EXACT(MTK_FPHY_ID_MT7628),
+		.name = "MediaTek MT7628 PHY",
+		.config_init = mt7628_phy_config_init,
+		.read_page = mt7628_phy_read_page,
+		.write_page = mt7628_phy_write_page,
+	},
+};
+
+module_phy_driver(mtk_soc_fe_phy_driver);
+static const struct mdio_device_id __maybe_unused mtk_soc_fe_phy_tbl[] = {
+	{ PHY_ID_MATCH_EXACT(MTK_FPHY_ID_MT7628) },
+	{ }
+};
+
+MODULE_DESCRIPTION("MediaTek SoC Fast Ethernet PHY driver");
+MODULE_AUTHOR("Joris Vaisvila <joey@tinyisr.com>");
+MODULE_LICENSE("GPL");
+
+MODULE_DEVICE_TABLE(mdio, mtk_soc_fe_phy_tbl);
-- 
2.53.0