ARM: dts: imx6sx: document SION necessity of ENET1_REF_CLK1

Signed-off-by: NChristoph Fritz <chf.fritz@googlemail.com>
Signed-off-by: NShawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx6sx-pinfunc.h

@@ -308,6 +308,20 @@
 #define MX6SX_PAD_ENET1_RX_CLK__VDEC_DEBUG_35                     0x008C 0x03D4 0x0000 0x8 0x0
 #define MX6SX_PAD_ENET1_RX_CLK__PCIE_CTRL_DEBUG_29                0x008C 0x03D4 0x0000 0x9 0x0
 #define MX6SX_PAD_ENET1_TX_CLK__ENET1_TX_CLK                      0x0090 0x03D8 0x0000 0x0 0x0
+/*
+ * SION bit is necessary for ENET1_REF_CLK1 (ENET2_REF_CLK2 untested) if it is
+ * used as clock output of IMX6SX_CLK_ENET_REF (ENET1_TX_CLK) to e.g. supply a
+ * PHY in RMII mode. This configuration is valid if:
+ *  - bit 1 in field IMX6SX_GPR1_FEC_CLOCK_PAD_DIR_MASK is set
+ *  - bit 1 in field IMX6SX_GPR1_FEC_CLOCK_MUX_SEL_MASK unset
+ * It seems to be a silicon bug that in this configuration ENET1_TX reference
+ * clock isn't provided automatically.  According to i.MX6SX reference manual
+ * (IOMUXC_GPR_GPR1 field descriptions: ENET1_CLK_SEL, Rev. 0 from 2/2015) it
+ * should be the case.
+ * So this might have unwanted side effects for other hardware units that are
+ * also connected to that pin and using respective function as input (e.g.
+ * UART1's DTR handling on MX6SX_PAD_ENET1_TX_CLK__UART1_DTR_B).
+ */
 #define MX6SX_PAD_ENET1_TX_CLK__ENET1_REF_CLK1                    0x0090 0x03D8 0x0760 0x1 0x1
 #define MX6SX_PAD_ENET1_TX_CLK__AUDMUX_AUD4_RXD                   0x0090 0x03D8 0x0644 0x2 0x1
 #define MX6SX_PAD_ENET1_TX_CLK__UART1_DTR_B                       0x0090 0x03D8 0x0000 0x3 0x0