ARM: imx25-pinfunc: document SION being important for MX25_PAD_SD1_CMD__SD1_CMD

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx25-pinfunc.h

@@ -421,6 +421,15 @@
 #define MX25_PAD_UART2_CTS__FEC_RX_ERR		0x18c 0x384 0x518 0x12 0x002
 #define MX25_PAD_UART2_CTS__GPIO_4_29		0x18c 0x384 0x000 0x15 0x000
 
+/*
+ * Removing the SION bit from MX25_PAD_SD1_CMD__SD1_CMD breaks detecting an SD
+ * card. According to the i.MX25 reference manual (e.g. Figure 23-2 in IMX25RM
+ * Rev. 2 from 01/2011) this pin is bidirectional. So it seems to be a silicon
+ * bug that configuring the SD1_CMD function doesn't enable the input path for
+ * this pin.
+ * This might have side effects for other hardware units that are connected to
+ * that pin and use the respective function as input.
+ */
 #define MX25_PAD_SD1_CMD__SD1_CMD		0x190 0x388 0x000 0x10 0x000
 #define MX25_PAD_SD1_CMD__CSPI2_MOSI		0x190 0x388 0x4a0 0x11 0x001
 #define MX25_PAD_SD1_CMD__FEC_RDATA2		0x190 0x388 0x50c 0x12 0x002