sunxi: dram: Configurable DQS gating window mode and delay

The hardware DQS gate training is a bit unreliable and does not
always find the best delay settings.

So we introduce a 32-bit 'dqs_gating_delay' variable, where each
byte encodes the DQS gating delay for each byte lane. The delay
granularity is 1/4 cycle.

Also we allow to enable the active DQS gating window mode, which
works better than the passive mode in practice. The DDR3 spec
says that there is a 0.9 cycles preamble and 0.3 cycle postamble.
The DQS window has to be opened during preamble and closed during
postamble. In the passive window mode, the gating window is opened
and closed by just using the gating delay settings. And because
of the 1/4 cycle delay granularity, accurately hitting the 0.3
cycle long postamble is a bit tough. In the active window mode,
the gating window is auto-closing with the help of monitoring
the DQS line, which relaxes the gating delay accuracy requirements.

But the hardware DQS gate training is still performed in the passive
window mode. It is a more strict test, which is reducing the results
variance compared to the training with active window mode.

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

@@ -136,6 +136,14 @@ static void mctl_itm_enable(void)
 	clrbits_le32(&dram->ccr, DRAM_CCR_ITM_OFF);
 }
 
+static void mctl_itm_reset(void)
+{
+	mctl_itm_disable();
+	udelay(1); /* ITM reset needs a bit of delay */
+	mctl_itm_enable();
+	udelay(1);
+}
+
 static void mctl_enable_dll0(u32 phase)
 {
 	struct sunxi_dram_reg *dram = (struct sunxi_dram_reg *)SUNXI_DRAMC_BASE;
@@ -356,6 +364,37 @@ static void mctl_setup_dram_clock(u32 clk, u32 mbus_clk)
 	udelay(22);
 }
 
+/*
+ * The data from rslrX and rdgrX registers (X=rank) is stored
+ * in a single 32-bit value using the following format:
+ *   bits [31:26] - DQS gating system latency for byte lane 3
+ *   bits [25:24] - DQS gating phase select for byte lane 3
+ *   bits [23:18] - DQS gating system latency for byte lane 2
+ *   bits [17:16] - DQS gating phase select for byte lane 2
+ *   bits [15:10] - DQS gating system latency for byte lane 1
+ *   bits [ 9:8 ] - DQS gating phase select for byte lane 1
+ *   bits [ 7:2 ] - DQS gating system latency for byte lane 0
+ *   bits [ 1:0 ] - DQS gating phase select for byte lane 0
+ */
+static void mctl_set_dqs_gating_delay(int rank, u32 dqs_gating_delay)
+{
+	struct sunxi_dram_reg *dram = (struct sunxi_dram_reg *)SUNXI_DRAMC_BASE;
+	u32 lane, number_of_lanes = mctl_get_number_of_lanes();
+	/* rank0 gating system latency (3 bits per lane: cycles) */
+	u32 slr = readl(rank == 0 ? &dram->rslr0 : &dram->rslr1);
+	/* rank0 gating phase select (2 bits per lane: 90, 180, 270, 360) */
+	u32 dgr = readl(rank == 0 ? &dram->rdgr0 : &dram->rdgr1);
+	for (lane = 0; lane < number_of_lanes; lane++) {
+		u32 tmp = dqs_gating_delay >> (lane * 8);
+		slr &= ~(7 << (lane * 3));
+		slr |= ((tmp >> 2) & 7) << (lane * 3);
+		dgr &= ~(3 << (lane * 2));
+		dgr |= (tmp & 3) << (lane * 2);
+	}
+	writel(slr, rank == 0 ? &dram->rslr0 : &dram->rslr1);
+	writel(dgr, rank == 0 ? &dram->rdgr0 : &dram->rdgr1);
+}
+
 static int dramc_scan_readpipe(void)
 {
 	struct sunxi_dram_reg *dram = (struct sunxi_dram_reg *)SUNXI_DRAMC_BASE;
@@ -618,7 +657,7 @@ unsigned long dramc_init(struct dram_para *para)
 	writel(para->emr2, &dram->emr2);
 	writel(para->emr3, &dram->emr3);
 
-	/* set DQS window mode */
+	/* disable drift compensation and set passive DQS window mode */
 	clrsetbits_le32(&dram->ccr, DRAM_CCR_DQS_DRIFT_COMP, DRAM_CCR_DQS_GATE);
 
 #ifdef CONFIG_SUN7I
@@ -631,11 +670,25 @@ unsigned long dramc_init(struct dram_para *para)
 
 	/* scan read pipe value */
 	mctl_itm_enable();
+
+	/* Hardware DQS gate training */
 	ret_val = dramc_scan_readpipe();
 
 	if (ret_val < 0)
 		return 0;
 
+	/* allow to override the DQS training results with a custom delay */
+	if (para->dqs_gating_delay)
+		mctl_set_dqs_gating_delay(0, para->dqs_gating_delay);
+
+	/* set the DQS gating window type */
+	if (para->active_windowing)
+		clrbits_le32(&dram->ccr, DRAM_CCR_DQS_GATE);
+	else
+		setbits_le32(&dram->ccr, DRAM_CCR_DQS_GATE);
+
+	mctl_itm_reset();
+
 	/* configure all host port */
 	mctl_configure_hostport();
 

arch/arm/include/asm/arch-sunxi/dram.h

@@ -88,6 +88,8 @@ struct dram_para {
 	u32 emr1;
 	u32 emr2;
 	u32 emr3;
+	u32 dqs_gating_delay;
+	u32 active_windowing;
 };
 
 #define DRAM_CCR_COMMAND_RATE_1T (0x1 << 5)