sunxi: dram: Fix CKE delay handling for sun4i/sun5i

Before driving the CKE pin (Clock Enable) high, the DDR3 spec requires
to wait for additional 500 us after the RESET pin is de-asserted.

The DRAM controller takes care of this delay by itself, using a
configurable counter in the SDR_IDCR register. This works in the same
way on sun4i/sun5i/sun7i hardware (even the default register value
0x00c80064 is identical). Except that the counter is ticking a bit
slower on sun7i (3 DRAM clock cycles instead of 2), resulting in
longer actual delays for the same settings.

This patch configures the SDR_IDCR register for all sun4i/sun5i/sun7i
SoC variants and not just for sun7i alone. Also an explicit udelay(500)
is added immediately after DDR3 reset for extra safety. This is a
duplicated functionality. But since we don't have perfect documentation,
it may be reasonable to play safe. Half a millisecond boot time increase
is not that significant. Boot time can be always optimized later.
Preferebly by the people, who have the hardware equipment to check the
actual signals on the RESET and CKE lines and verify all the timings.

The old code did not configure the SDR_IDCR register for sun4i/sun5i,
but performed the DDR3 reset very early for sun4i/sun5i. This resulted
in a larger time gap between the DDR3 reset and the DDR3 initialization
steps and reduced the chances of CKE delay timing violation to cause
real troubles.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

arch/arm/cpu/armv7/sunxi/dram.c

@@ -78,6 +78,19 @@ static void mctl_ddr3_reset(void)
 		udelay(200);
 		setbits_le32(&dram->mcr, DRAM_MCR_RESET);
 	}
+	/* After the RESET pin is de-asserted, the DDR3 spec requires to wait
+	 * for additional 500 us before driving the CKE pin (Clock Enable)
+	 * high. The duration of this delay can be configured in the SDR_IDCR
+	 * (Initialization Delay Configuration Register) and applied
+	 * automatically by the DRAM controller during the DDR3 initialization
+	 * step. But SDR_IDCR has limited range on sun4i/sun5i hardware and
+	 * can't provide sufficient delay at DRAM clock frequencies higher than
+	 * 524 MHz (while Allwinner A13 supports DRAM clock frequency up to
+	 * 533 MHz according to the datasheet). Additionally, there is no
+	 * official documentation for the SDR_IDCR register anywhere, and
+	 * there is always a chance that we are interpreting it wrong.
+	 * Better be safe than sorry, so add an explicit delay here. */
+	udelay(500);
 }
 
 static void mctl_set_drive(void)
@@ -382,6 +395,40 @@ static void mctl_disable_power_save(void)
 	writel(0x16510000, &dram->ppwrsctl);
 }
 
+/*
+ * After the DRAM is powered up or reset, the DDR3 spec requires to wait at
+ * least 500 us before driving the CKE pin (Clock Enable) high. The dram->idct
+ * (SDR_IDCR) register appears to configure this delay, which gets applied
+ * right at the time when the DRAM initialization is activated in the
+ * 'mctl_ddr3_initialize' function.
+ */
+static void mctl_set_cke_delay(void)
+{
+	struct sunxi_dram_reg *dram = (struct sunxi_dram_reg *)SUNXI_DRAMC_BASE;
+
+	/* The CKE delay is represented in DRAM clock cycles, multiplied by N
+	 * (where N=2 for sun4i/sun5i and N=3 for sun7i). Here it is set to
+	 * the maximum possible value 0x1ffff, just like in the Allwinner's
+	 * boot0 bootloader. The resulting delay value is somewhere between
+	 * ~0.4 ms (sun5i with 648 MHz DRAM clock speed) and ~1.1 ms (sun7i
+	 * with 360 MHz DRAM clock speed). */
+	setbits_le32(&dram->idcr, 0x1ffff);
+}
+
+/*
+ * This triggers the DRAM initialization. It performs sending the mode registers
+ * to the DRAM among other things. Very likely the ZQCL command is also getting
+ * executed (to do the initial impedance calibration on the DRAM side of the
+ * wire). The memory controller and the PHY must be already configured before
+ * calling this function.
+ */
+static void mctl_ddr3_initialize(void)
+{
+	struct sunxi_dram_reg *dram = (struct sunxi_dram_reg *)SUNXI_DRAMC_BASE;
+	setbits_le32(&dram->ccr, DRAM_CCR_INIT);
+	await_completion(&dram->ccr, DRAM_CCR_INIT);
+}
+
 unsigned long dramc_init(struct dram_para *para)
 {
 	struct sunxi_dram_reg *dram = (struct sunxi_dram_reg *)SUNXI_DRAMC_BASE;
@@ -459,10 +506,7 @@ unsigned long dramc_init(struct dram_para *para)
 	writel(reg_val, &dram->zqcr0);
 #endif
 
-#ifdef CONFIG_SUN7I
-	/* Set CKE Delay to about 1ms */
-	setbits_le32(&dram->idcr, 0x1ffff);
-#endif
+	mctl_set_cke_delay();
 
 #ifdef CONFIG_SUN7I
 	mctl_ddr3_reset();
@@ -527,9 +571,8 @@ unsigned long dramc_init(struct dram_para *para)
 	if (para->tpr4 & 0x1)
 		setbits_le32(&dram->ccr, DRAM_CCR_COMMAND_RATE_1T);
 #endif
-	/* reset external DRAM */
-	setbits_le32(&dram->ccr, DRAM_CCR_INIT);
-	await_completion(&dram->ccr, DRAM_CCR_INIT);
+	/* initialize external DRAM */
+	mctl_ddr3_initialize();
 
 	/* scan read pipe value */
 	mctl_itm_enable();