powerpc/mpc8xxx: Enable entering DDR debugging by key press

Using environmental variable "ddr_interactive" to activate interactive DDR
debugging seomtiems is not enough. For example, after updating SPD with a
valid but wrong image, u-boot won't come up due to wrong DDR configuration.
By enabling key press method, we can enter debug mode to have a chance to
boot without using other tools to recover the board.

CONFIG_FSL_DDR_INTERACTIVE needs to be defined in header file. To enter the
debug mode by key press, press key 'd' shortly after reset, like one would
do to abort auto booting. It is fixed to lower case 'd' at this moment.

Signed-off-by: York Sun <yorksun@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>

arch/powerpc/cpu/mpc8xxx/ddr/main.c

@@ -532,9 +532,11 @@ phys_size_t fsl_ddr_sdram(void)
 
 	/* Compute it once normally. */
 #ifdef CONFIG_FSL_DDR_INTERACTIVE
-	if (getenv("ddr_interactive"))
+	if (getenv("ddr_interactive")) {
 		total_memory = fsl_ddr_interactive(&info);
-	else
+	} else if (tstc() && (getc() == 'd')) {	/* we got a key press of 'd' */
+		total_memory = fsl_ddr_interactive(&info);
+	} else
 #endif
 		total_memory = fsl_ddr_compute(&info, STEP_GET_SPD, 0);
 

doc/README.fsl-ddr

@@ -268,6 +268,13 @@ be activated by saving an environment variable "ddr_interactive". The value
 doesn't matter. Once activated, U-boot prompts "FSL DDR>" before enabling DDR
 controller. The available commands can be seen by typing "help".
 
+Another way to enter debug mode without using environment variable is to send
+a key press during boot, like one would do to abort auto boot. To save booting
+time, no additioal delay is added so the window to send the key press is very
+short. For example, user can send the key press using reset command followed by
+hitting enter key twice. In case of power on reset, user can keep hitting any
+key while applying the power.
+
 The example flow of using interactive debugging is
 type command "compute" to calculate the parameters from the default
 type command "print" with arguments to show SPD, options, registers