[U-Boot] [PATCH v2 15/16] sunxi: dram: Derive write recovery delay from DRAM clock speed

[Siarhei Siamashka]

The write recovery time is 15ns for all JEDEC DDR3 speed bins. And
instead of hardcoding it to 10 cycles, it is possible to set tighter
timings based on accurate calculations. For example, DRAM clock
frequencies up to 533MHz need only 8 cycles for write recovery.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka at gmail.com>
Acked-by: Ian Campbell <ijc at hellion.org.uk>
---
Changes for v2:
   - no changes

 arch/arm/cpu/armv7/sunxi/dram.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/arch/arm/cpu/armv7/sunxi/dram.c b/arch/arm/cpu/armv7/sunxi/dram.c
index 0bbb10d..2ad685b 100644
--- a/arch/arm/cpu/armv7/sunxi/dram.c
+++ b/arch/arm/cpu/armv7/sunxi/dram.c
@@ -451,6 +451,21 @@ static void dramc_set_autorefresh_cycle(u32 clk, u32 density)
 	writel(DRAM_DRR_TREFI(tREFI) | DRAM_DRR_TRFC(tRFC), &dram->drr);
 }
 
+/* Calculate the value for A11, A10, A9 bits in MR0 (write recovery) */
+static u32 ddr3_write_recovery(u32 clk)
+{
+	u32 twr_ns = 15; /* DDR3 spec says that it is 15ns for all speed bins */
+	u32 twr_ck = (twr_ns * clk + 999) / 1000;
+	if (twr_ck < 5)
+		return 1;
+	else if (twr_ck <= 8)
+		return twr_ck - 4;
+	else if (twr_ck <= 10)
+		return 5;
+	else
+		return 6;
+}
+
 /*
  * If the dram->ppwrsctl (SDR_DPCR) register has the lowest bit set to 1, this
  * means that DRAM is currently in self-refresh mode and retaining the old
@@ -647,7 +662,7 @@ unsigned long dramc_init(struct dram_para *para)
 	reg_val |= DRAM_MR_POWER_DOWN;
 #endif
 	reg_val |= DRAM_MR_CAS_LAT(para->cas - 4);
-	reg_val |= DRAM_MR_WRITE_RECOVERY(0x5);
+	reg_val |= DRAM_MR_WRITE_RECOVERY(ddr3_write_recovery(para->clock));
 	writel(reg_val, &dram->mr);
 
 	writel(para->emr1, &dram->emr);
-- 
1.8.3.2