edb93xx: Fix SDRAM initialization

edb93xx: Fix SDRAM initialization by issuing a precharge all command before
forcing the precharge and select mode register update mode before programming
the mode registers. Write to the SDRAM banks in order to force a precharge,
reading causes the edb93xx boards to hang

Signed-off-by: Matthias Kaehlcke <matthias@kaehlcke.net>

board/edb93xx/sdram_cfg.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2009 Matthias Kaehlcke <matthias@kaehlcke.net>
+ * Copyright (C) 2009, 2010 Matthias Kaehlcke <matthias@kaehlcke.net>
  *
  * Copyright (C) 2006 Dominic Rath <Dominic.Rath@gmx.de>
  *
@@ -30,9 +30,9 @@
 		(SDRAM_BASE_ADDR | SDRAM_BANK_SEL_##bank | SDRAM_MODE_REG_VAL))
 
 #define PRECHARGE_BANK(bank)		(*(volatile uint32_t *)		\
-				(SDRAM_BASE_ADDR | SDRAM_BANK_SEL_##bank))
+		(SDRAM_BASE_ADDR | SDRAM_BANK_SEL_##bank)) = 0
 
-static void force_precharge(void);
+static void precharge_all_banks(void);
 static void setup_refresh_timer(void);
 static void program_mode_registers(void);
 
@@ -47,7 +47,7 @@ void sdram_cfg(void)
 
 	early_udelay(200);
 
-	force_precharge();
+	precharge_all_banks();
 
 	setup_refresh_timer();
 
@@ -57,19 +57,37 @@ void sdram_cfg(void)
 	writel(GLCONFIG_CKE, &sdram->glconfig);
 }
 
-static void force_precharge(void)
+static void precharge_all_banks(void)
 {
+	struct sdram_regs *sdram = (struct sdram_regs *)SDRAM_BASE;
+
+	/* Issue PRECHARGE ALL commands */
+	writel(GLCONFIG_INIT | GLCONFIG_CKE, &sdram->glconfig);
+
 	/*
-	 * Errata most EP93xx revisions say that PRECHARGE ALL isn't always
-	 * issued.
+	 * Errata of most EP93xx revisions say that PRECHARGE ALL isn't always
+	 * issued
 	 *
-	 * Do a read from each bank to make sure they're precharged
+	 * Cirrus proposes a workaround which consists in performing a read from
+	 * each bank to force the precharge. This causes some boards to hang.
+	 * Writing to the SDRAM banks instead of reading has the same
+	 * side-effect (the SDRAM controller issues the necessary precharges),
+	 * but is known to work on all supported boards
 	 */
 
 	PRECHARGE_BANK(0);
+
+#if (CONFIG_NR_DRAM_BANKS >= 2)
 	PRECHARGE_BANK(1);
+#endif
+
+#if (CONFIG_NR_DRAM_BANKS >= 3)
 	PRECHARGE_BANK(2);
+#endif
+
+#if (CONFIG_NR_DRAM_BANKS == 4)
 	PRECHARGE_BANK(3);
+#endif
 }
 
 static void setup_refresh_timer(void)
@@ -101,6 +119,11 @@ static void setup_refresh_timer(void)
 
 static void program_mode_registers(void)
 {
+	struct sdram_regs *sdram = (struct sdram_regs *)SDRAM_BASE;
+
+	/* Select mode register update mode */
+	writel(GLCONFIG_MRS | GLCONFIG_CKE, &sdram->glconfig);
+
 	/*
 	 * The mode registers are programmed by performing a read from each
 	 * SDRAM bank. The value of the address that is read defines the value