[U-Boot] [PATCH] ARM: mx6: ddr: Add write leveling correction code

[Stefano Babic]

On 30/03/2018 18:10, Eric Nelson wrote:
> Hi Marek,
> 
> Thanks for this update and the detailed notes.
> 
> On 03/29/2018 06:04 PM, Marek Vasut wrote:
>> When the DDR calibration is enabled, a situation may happen that it
>> will fail on a few select boards out of a whole production lot. In
>> particular, after the first write leveling stage, the MPWLDECTRLx
>> registers will contain a value 0x1nn , for nn usually being 0x7f or
>> slightly lower.
>>
>> What this means is that the HW write leveling detected that the DQS
>> rising edge on one or more bundles arrives slightly _after_ CLK and
>> therefore when the DDR DRAM samples CLK on the DQS rising edge, the
>> CLK signal is already high (cfr. AN4467 rev2 Figure 7 on page 18).
>>
>> The HW write leveling then ends up adding almost an entire cycle (thus
>> the 0x17f) to the DQS delay, which indeed aligns it, but also triggers
>> subsequent calibration failure in DQS gating due to this massive offset.
>>
>> There are two observations here:
>> - If the MPWLDECTRLx value is corrected from 0x17f to 0x0 , then the
>>    DQS gating passes, the entire calibration passes as well and the
>>    DRAM is perfectly stable even under massive load.
>> - When using the NXP DRAM calibrator for iMX6/7, the value 0x17f or so
>>    in MPWLDECTRx register is not there, but it is replaced by 0x0 as one
>>    would expect.
>>
>> Someone from NXP finally explains why, quoting [1]:
>>
>>      "
>>      Having said all that, the DDR Stress Test does something that we
>>      do not advertise to the users. The Stress Test iself looks at the
>>      values of the MPWLDECTRL0/1 fields before reporting results, and
>>      if it sees any filed with a value greater than 200/256 delay
>>      (reported as half-cycle = 0x1 and ABS_OFFSET > 0x48), the DDR
>>      Stress test will reset the Write Leveling delay for this lane
>>      to 0x000 and not report it in the log.
>>
>>      The reason that the DDR Stress test does this is because a delay
>>      of more than 78% a clock cycle means that the DQS edge is arriving
>>      within the JEDEC tolerence of 25% of the clock edge. In most cases,
>>      DQS is arriving < 5% tCK of the SDCLK edge in the early case, and
>>      it does not make sense to delay the DQS strobe almost a full clock
>>      cycle and add extra latency to each Write burst just to make the
>>      two edges align exactly. In this case, we are guilty of making a
>>      decision for the customer without telling them we are doing it so
>>      that we don't have to provide the above explanation to every
>> customer.
>>      They don't need to know it.
>>      "
>>
>> This patch adds the correction described above, that is if the MPWLDECTRx
>> value is over 0x148, the value is corrected back to 0x0.
>>
>> [1] https://community.nxp.com/thread/456246
>>
>> Signed-off-by: Marek Vasut <marex at denx.de>
>> Cc: Stefano Babic <sbabic at denx.de>
>> ---
>>   arch/arm/mach-imx/mx6/ddr.c | 24 ++++++++++++++++++++++++
>>   1 file changed, 24 insertions(+)
>>
>> diff --git a/arch/arm/mach-imx/mx6/ddr.c b/arch/arm/mach-imx/mx6/ddr.c
>> index 43b77cfa41..6e5e40dd1a 100644
>> --- a/arch/arm/mach-imx/mx6/ddr.c
>> +++ b/arch/arm/mach-imx/mx6/ddr.c
>> @@ -85,6 +85,23 @@ static void modify_dg_result(u32 *reg_st0, u32
>> *reg_st1, u32 *reg_ctrl)
>>       writel(val_ctrl, reg_ctrl);
>>   }
>>   +static void correct_mpwldectr_result(void *reg)
>> +{
>> +    /* Limit is 200/256 of CK, which is WL_HC_DELx | 0x48. */
>> +    const unsigned int limit = 0x148;
>> +    u32 val = readl(reg);
>> +    u32 old = val;
>> +
> 
> Nit: I think "val &= 0xffff0000" would be slightly easier to read
> instead of the "<< 16":

I think it is just a question of taste....

> 
>> +    if ((val & 0x17f) > limit)
>> +        val &= 0xffff << 16;
>> +
>> +    if (((val >> 16) & 0x17f) > limit)
>> +        val &= 0xffff;
>> +
>> +    if (old != val)
>> +        writel(val, reg);
>> +}
>> +
>>   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;
>> @@ -176,6 +193,13 @@ int mmdc_do_write_level_calibration(struct
>> mx6_ddr_sysinfo const *sysinfo)
>>           errors |= 4;
>>       }
>>   +    correct_mpwldectr_result(&mmdc0->mpwldectrl0);
>> +    correct_mpwldectr_result(&mmdc0->mpwldectrl1);
>> +    if (sysinfo->dsize == 2) {
>> +        correct_mpwldectr_result(&mmdc1->mpwldectrl0);
>> +        correct_mpwldectr_result(&mmdc1->mpwldectrl1);
>> +    }
>> +
>>       /*
>>        * User should issue MRS command to exit write leveling mode
>>        * through Load Mode Register command
>>
> 
> Otherwise,
> 
> Reviewed-by: Eric Nelson <eric at nelint.com>
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