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

[Marek Vasut]

On 4/4/22 16:53, Francesco Dolcini wrote:
> 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).

Can the tXPR ever be larger than 500us ?