Hi Marek, On Thu, Mar 29, 2018 at 10:04 PM, Marek Vasut <ma...@denx.de> 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 <ma...@denx.de> > Cc: Stefano Babic <sba...@denx.de>
Thanks for the fix and for the detailed commit log! Reviewed-by: Fabio Estevam <fabio.este...@nxp.com> _______________________________________________ U-Boot mailing list U-Boot@lists.denx.de https://lists.denx.de/listinfo/u-boot
