[RFC PATCH 1/3] mx6: ddr: Restore ralat/walat in write level calibration

[Francesco Dolcini <francesco.dolcini@toradex.com>]

The current DDR write level calibration routine always overwrite
the ralat/walat fields to their maximum value, just save
the existing values at the beginning of the calibration routine
and restore it at the end.

In case the delay is estimated by the user to be more than one cycle the
walat should be configured according to that, this is not
automatically done.  From the i.MX6 RM:

   The user should read the results of the associated delay-line at
   MPWLDECTRL#[WL_DL_ABS_OFFSET#] and in case the user estimates that the
   reasonable delay may be above 1 cycle then the user should indicate it at
   MPWLDECTRL#[WL_CYC_DEL#]. Moreover the user should indicate it in
   MDMISC[WALAT] field. For example, if the result of the write leveling calibration
   is 100/256 parts of a cycle, but the user estimates that the delay is above 2 cycles
   then MPWLDECTRL#[WL_CYC_DEL#] should be configured to 2, so the total
   delay will be 2 and 100/256 parts of a cycle

Probably it would just possible to not overwrite the mdmisc register in
the first place, since this is not present in the write_level_calib() example
in NXP AN4467 [1] nor in the i.MX6 RM (44.11.6.1 Hardware Write Leveling
Calibration).

Fixes: d339f16911c7 ("arm: imx6: Add DDR3 calibration code for MX6 Q/D/DL")
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
---
 arch/arm/mach-imx/mx6/ddr.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/arm/mach-imx/mx6/ddr.c b/arch/arm/mach-imx/mx6/ddr.c
index f872bfdab315..08e2f0f130a6 100644
--- a/arch/arm/mach-imx/mx6/ddr.c
+++ b/arch/arm/mach-imx/mx6/ddr.c
@@ -108,7 +108,7 @@ int mmdc_do_write_level_calibration(struct mx6_ddr_sysinfo const *sysinfo)
 {
 	struct mmdc_p_regs *mmdc0 = (struct mmdc_p_regs *)MMDC_P0_BASE_ADDR;
 	struct mmdc_p_regs *mmdc1 = (struct mmdc_p_regs *)MMDC_P1_BASE_ADDR;
-	u32 esdmisc_val, zq_val;
+	u32 esdmisc_val, zq_val, mdmisc_val;
 	u32 errors = 0;
 	u32 ldectrl[4] = {0};
 	u32 ddr_mr1 = 0x4;
@@ -131,6 +131,9 @@ int mmdc_do_write_level_calibration(struct mx6_ddr_sysinfo const *sysinfo)
 	/* disable Adopt power down timer */
 	setbits_le32(&mmdc0->mapsr, 0x1);
 
+	/* Save old RALAT and WALAT values */
+	mdmisc_val = readl(&mmdc0->mdmisc);
+
 	debug("Starting write leveling calibration.\n");
 
 	/*
@@ -217,6 +220,9 @@ int mmdc_do_write_level_calibration(struct mx6_ddr_sysinfo const *sysinfo)
 	writel(esdmisc_val, &mmdc0->mdref);
 	writel(zq_val, &mmdc0->mpzqhwctrl);
 
+	/* restore walat/ralat */
+	writel(mdmisc_val, &mmdc0->mdmisc);
+
 	debug("\tMMDC_MPWLDECTRL0 after write level cal: 0x%08x\n",
 	      readl(&mmdc0->mpwldectrl0));
 	debug("\tMMDC_MPWLDECTRL1 after write level cal: 0x%08x\n",
-- 
2.25.1