[U-Boot] [RFC] am33xx: add 600us wait in DDR3 initialization sequence

[Doublesin, James]

Hi Samuel, I don't think these changes are necessary.  

I think your concern about the initialization steps is not correct.  The first setting of the REF_CTRL (before SDRAM_CONFIG write) will be used by the EMIF controller to determine the delay between the rising edge of DDR_RESETn and DDR_CKE.  We typically write a value of 0x3100 in REF_CTRL to achieve this, which comes from this formula:

  (16*SDRAM_REF_CTRL)/400MHz > 500us

This value will change for different frequencies, but for AM335x and AM437x, the max freq is 400MHz.  When SDRAM_CONFIG is written, the EMIF controller will kick off a H/W sequence which will configure the memory, and will use this setting for the proper RESET to CKE timing.  Once that is finished, another write to REF_CTRL to set the proper refresh rate will be necessary.

A separate issue which we have seen does require a delay, however.  We have seen some timeout errors reported on the L3 bus as a result of this EMIF sequence.  This error does not result in any functional problem with the EMIF initialization, but does result in one of the L3 error flags being set.  We have determined that this is a result of an EMIF access immediately following the SDRAM_CONFIG write.  The EMIF kicks back an error while the H/W initialization sequence is still active, thus a delay is required to eliminate this bus error.  Again, functionally the EMIF sequence will work without this delay, but to avoid the error flag, the delay is needed.

So to summarize, the sequence should be:

-Configure all other EMIF registers
-Write SDRAM_REF_CTRL=0x3100
-Write SDRAM_CONFIG with appropriate value to kick off H/W initialization of the DDR
-wait 1ms (to avoid L3 timeout error)
-Write SDRAM_REF_CTRL with refresh rate value for your DDR

Regards ,
James Doublesin
-----Original Message-----
From: Egli, Samuel [mailto:samuel.egli at siemens.com] 
Sent: Tuesday, July 28, 2015 9:32 AM
To: 'trini at konsulko.com'; Doublesin, James; Balbi, Felipe
Cc: 'u-boot at lists.denx.de'; 'hs at denx.de'; 'Stefan Roese'; Meier, Roger; Senn, Joerg; Belogolov, Oleg
Subject: [RFC] am33xx: add 600us wait in DDR3 initialization sequence

Hi all,
me again. I was wondering, if somebody of you has time to check these changes? I would appreciate some feedback. Ultimately, it should also go upstream but my focus here is reviewing the content of these changes.

Thanks,

Sam

> -----Original Message-----
> From: Egli, Samuel
> Sent: Freitag, 17. Juli 2015 13:47
> To: trini at konsulko.com; doublesin at ti.com; balbi at ti.com
> Cc: u-boot at lists.denx.de; hs at denx.de; Stefan Roese; Meier, Roger; 
> Senn, Joerg
> Subject: [RFC] am33xx: add 600us wait in DDR3 initialization sequence
> 
> Hi all,
> the current implementation in ./arch/arm/cpu/armv7/am33xx/emif4.c does 
> not respect the the initialization steps as documented in the DDR3 
> specification [1]. We noticed that for our am335x based siemens boards 
> a delay after config_ddr(...) call was necessary otherwise boot would fail.
> See 61159b76844437bf9004c3a38b5a4ff1a24860d5
> commit that added a 2ms delay which was merly combating symptoms.
> 
> This is because SDRAM wouldn't be ready yet. We didn't realy know why 
> it should be necessary but the delay did the job. However, we figured 
> out now that an important wait time is not respected, specifically, step 4:
> 
> "After RESET# transitions HIGH, wait 500µs (minus one clock) with CKE LOW"
> [1]
> 
> We also noticed that it is vendor specific and some SDRAM wouldn't 
> bother if
> 500 us wait is done or not.
> 
> See below the patch that guarantees that we have a 600µs wait time 
> before the clock enable gets enabled. Maybe a comment on the main steps:
> 
> (1) The first time we call config_sdram the CKE controll is still refused
>     EMIF/DDR PHY. But it has the effect that the init sequence is started
>     anyway, and as a consequence puts RESET to high which is the sole goal
>     of this line.
> 
> In (2) + (3) we wait 600us. 100us more to be on the safe side and then 
> give control to EMIF/DDR PHY.
> 
> (4) Now, that the wait time is ensured and CKE can be used by the 
> EMIF/DDR PHY
>     we trigger the real sdram configuration sequence.
> 
> 
> This patch works fine for us but I'm not sure if some revising of the
> EMIF_4D5 case is necessary, too. With this patch no delay after
> config_ddr(...) is necessary anymore.
> 
> One thing that I cannot explain well, however, is why no issue was 
> observed with other TI boards. A possible explenation is that some 
> delay stems from code execution time that is done after sdram config 
> before loading u-boot.
> 
> But maybe it has also to do with different DDR3 vendors being used. 
> Our observation was that Samsung SDRAM is more likely to not work. And 
> that smaller sized SDRAM is less/not prone to ignoring the spec. I.e. 
> SDRAM with only 128 MB didn't cause any problem. But 256/512 MB from 
> Samsung would not work.
> 
> Kind regards
> 
> Sam
> 
> [1] 
> http://www.micron.com/products/datasheets/%7B2ADCEEB1-307F-4B37-99C9-
> 7302CAA8BC5C%7D?page=8#topic=c_Initialization
> 
> 
> ---
>  arch/arm/cpu/armv7/am33xx/emif4.c |   18 +++++++++++++-----
>  1 file changed, 13 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm/cpu/armv7/am33xx/emif4.c
> b/arch/arm/cpu/armv7/am33xx/emif4.c
> index 9cf816c..084b45a 100644
> --- a/arch/arm/cpu/armv7/am33xx/emif4.c
> +++ b/arch/arm/cpu/armv7/am33xx/emif4.c
> @@ -109,10 +109,6 @@ void config_ddr(unsigned int pll, const struct 
> ctrl_ioregs *ioregs,
>  	config_ddr_data(data, nr);
>  #ifdef CONFIG_AM33XX
>  	config_io_ctrl(ioregs);
> -
> -	/* Set CKE to be controlled by EMIF/DDR PHY */
> -	writel(DDR_CKE_CTRL_NORMAL, &ddrctrl->ddrckectrl);
> -
>  #endif
>  #ifdef CONFIG_AM43XX
>  	writel(readl(&cm_device->cm_dll_ctrl) & ~0x1, &cm_device-
> >cm_dll_ctrl); @@ -131,9 +127,21 @@ void config_ddr(unsigned int pll,
> const struct ctrl_ioregs *ioregs,
>  	/* Program EMIF instance */
>  	config_ddr_phy(regs, nr);
>  	set_sdram_timings(regs, nr);
> +
>  	if (get_emif_rev(EMIF1_BASE) == EMIF_4D5)
>  		config_sdram_emif4d5(regs, nr);
> -	else
> +	else {
> +		/* (1) Set reset signal with CKE still off */
> +		config_sdram(regs, nr);
> +
> +		/* (2) Add 600us delay between Reset and CKE */
> +		udelay(600);
> +
> +		/* (3) Set CKE to be controlled by EMIF/DDR PHY */
> +		writel(DDR_CKE_CTRL_NORMAL, &ddrctrl->ddrckectrl);
> +
> +		/* (4) Configure sdram */
>  		config_sdram(regs, nr);
> +	}
>  }
>  #endif
> --
> 1.7.10.4