[PATCH net v3] net: stmmac: thead: Enable TX clock before MAC initialization

[Yao Zi <ziyao@disroot.org>]

The clk_tx_i clock must be supplied to the MAC for successful
initialization. On TH1520 SoC, the clock is provided by an internal
divider configured through GMAC_PLLCLK_DIV register when using RGMII
interface. However, currently we don't setup the divider before
initialization of the MAC, resulting in DMA reset failures if the
bootloader/firmware doesn't enable the divider,

[    7.839601] thead-dwmac ffe7060000.ethernet eth0: Register MEM_TYPE_PAGE_POOL RxQ-0
[    7.938338] thead-dwmac ffe7060000.ethernet eth0: PHY [stmmac-0:02] driver [RTL8211F Gigabit Ethernet] (irq=POLL)
[    8.160746] thead-dwmac ffe7060000.ethernet eth0: Failed to reset the dma
[    8.170118] thead-dwmac ffe7060000.ethernet eth0: stmmac_hw_setup: DMA engine initialization failed
[    8.179384] thead-dwmac ffe7060000.ethernet eth0: __stmmac_open: Hw setup failed

Let's simply write GMAC_PLLCLK_DIV_EN to GMAC_PLLCLK_DIV to enable the
divider before MAC initialization. Note that for reconfiguring the
divisor, the divider must be disabled first and re-enabled later to make
sure the new divisor take effect.

The exact clock rate doesn't affect MAC's initialization according to my
test. It's set to the speed required by RGMII when the linkspeed is
1Gbps and could be reclocked later after link is up if necessary.

Fixes: 33a1a01e3afa ("net: stmmac: Add glue layer for T-HEAD TH1520 SoC")
Signed-off-by: Yao Zi <ziyao@disroot.org>
---

Note that the DMA reset failures cannot be reproduced with the vendor
U-Boot, which always enables the divider, regardless whether the port is
used[1].

[1]: https://github.com/revyos/thead-u-boot/blob/93ff49d9f5bbe7942f727ab93311346173506d27/board/thead/light-c910/light.c#L581-L582

Changed from v2
- Explain the special process for changing divider's rate in commit
  message
- Fix the typo where ';' is mistyped as ','
- Link to v2: https://lore.kernel.org/all/20250808103447.63146-2-ziyao@disroot.org/
Changed from v1
- Initialize the divisor to a well-known value (producing the clock rate
  required by RGMII link at 1Gbps)
- Write zero to GMAC_PLLCLK_DIV before writing the configuration, as
  required by the TRM
- Link to v1: https://lore.kernel.org/netdev/20250801094507.54011-1-ziyao@disroot.org/

 drivers/net/ethernet/stmicro/stmmac/dwmac-thead.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/dwmac-thead.c b/drivers/net/ethernet/stmicro/stmmac/dwmac-thead.c
index f2946bea0bc2..6c6c49e4b66f 100644
--- a/drivers/net/ethernet/stmicro/stmmac/dwmac-thead.c
+++ b/drivers/net/ethernet/stmicro/stmmac/dwmac-thead.c
@@ -152,7 +152,7 @@ static int thead_set_clk_tx_rate(void *bsp_priv, struct clk *clk_tx_i,
 static int thead_dwmac_enable_clk(struct plat_stmmacenet_data *plat)
 {
 	struct thead_dwmac *dwmac = plat->bsp_priv;
-	u32 reg;
+	u32 reg, div;
 
 	switch (plat->mac_interface) {
 	case PHY_INTERFACE_MODE_MII:
@@ -164,6 +164,13 @@ static int thead_dwmac_enable_clk(struct plat_stmmacenet_data *plat)
 	case PHY_INTERFACE_MODE_RGMII_RXID:
 	case PHY_INTERFACE_MODE_RGMII_TXID:
 		/* use pll */
+		div = clk_get_rate(plat->stmmac_clk) / rgmii_clock(SPEED_1000);
+		reg = FIELD_PREP(GMAC_PLLCLK_DIV_EN, 1) |
+		      FIELD_PREP(GMAC_PLLCLK_DIV_NUM, div);
+
+		writel(0, dwmac->apb_base + GMAC_PLLCLK_DIV);
+		writel(reg, dwmac->apb_base + GMAC_PLLCLK_DIV);
+
 		writel(GMAC_GTXCLK_SEL_PLL, dwmac->apb_base + GMAC_GTXCLK_SEL);
 		reg = GMAC_TX_CLK_EN | GMAC_TX_CLK_N_EN | GMAC_TX_CLK_OUT_EN |
 		      GMAC_RX_CLK_EN | GMAC_RX_CLK_N_EN;
-- 
2.50.1