[RFC PATCH 2/3] mx6: ddr: Wait before issuing the first MRS cmd

[Francesco Dolcini]

Hello Marek,
thanks for your review.

On Mon, Apr 04, 2022 at 03:39:35PM +0200, Marek Vasut wrote:
> On 4/4/22 10:51, Francesco Dolcini wrote:
> > Wait 1ms before issuing the first MRS command to write DDR3 Mode
> > registers.
> > 
> > There is a requirement to wait minimum of Reset CKE Exit time, tXPR,
> > with tXPR = max(tXS, 5tCK) and to wait 500 useconds after reset is
> > de-asserted. It seems that for some reason this is not enforced by the
> > MMDC controller, despite MMDCx_MDOR RST_to_CKE and tXPR being correctly
> > configured.
> > 
> > Without this change we experienced random memory initialization failures
> > with about 2% boot failure rate on specific problematic boards, after
> > this change we were able to do more than 10.000 power-cycle without a
> > single failure.
> > 
> > Fixes: fe0f7f7842e1 ("mx6: add mmdc configuration for MX6Q/MX6DL")
> > Signed-off-by: Francesco Dolcini <francesco.dolcini at toradex.com>
> > ---
> >   arch/arm/mach-imx/mx6/ddr.c | 2 ++
> >   1 file changed, 2 insertions(+)
> > 
> > diff --git a/arch/arm/mach-imx/mx6/ddr.c b/arch/arm/mach-imx/mx6/ddr.c
> > index 08e2f0f130a6..7b3d620094c4 100644
> > --- a/arch/arm/mach-imx/mx6/ddr.c
> > +++ b/arch/arm/mach-imx/mx6/ddr.c
> > @@ -1526,6 +1526,8 @@ void mx6_ddr3_cfg(const struct mx6_ddr_sysinfo *sysinfo,
> >   			((sysinfo->ncs == 2) ? 1 : 0) << 30; /* SDE_1 for CS1 */
> >   	/* Step 8: Write Mode Registers to Init DDR3 devices */
> > +	mdelay(1); /* Wait before issuing the first MRS command
> > +		      (tXPR / 500us CKE delay after reset deassertion) */
> 
> Should we infer this delay from tXPR instead ?

I could just delay(tXPR + 500us) and do the exact worst case delay.

However I wonder if it is worth doing it, the 1ms delay works in
practice, it is big enough to be correct in any case, but small enough
not to be a concern on the boot time.

Please note that I do not know which timing is violated here
(tXPR, the 500us after reset de-assertion or both of them).

Francesco