[U-Boot] How do ARM platform initialize DDR?

[York Sun <yorksun@freescale.com>]

On 09/19/2013 02:33 PM, Tom Rini wrote:
> On 09/19/2013 05:08 PM, York Sun wrote:
>> On 09/19/2013 01:57 PM, Tom Rini wrote:
>>> On Tue, Sep 17, 2013 at 08:59:24AM -0700, York Sun wrote:
>>>
>>>> Albert,
>>>>
>>>> Pardon me if this is a dumb question. I have been working on
>>>> powerpc platforms in the past. Now we (the developers I work
>>>> with) are exploring ARM cores. I am searching how memory is
>>>> initialized and found different solutions. Some platforms have
>>>> memory ready before u-boot even starts, some simply write to a
>>>> set of registers. I understand many platforms don't share the
>>>> IP of DDR controller. I am wondering if there is generic DDR
>>>> driver used by many ARM platforms, like the one we have for 
>>>> powerpc/mpc85xx SoCs.
>>>
>>> Thinking back, as a rule of thumb, PowerPC has SPD I2C data 
>>> available, usually.  That's not the rule for ARM.  One of a few
>>> choices happen: 1) ROM sets up DDR. 2) U-Boot/SPL sets up the DDR
>>> controller.
>>>
> 
>> So for ARM platforms, the majority don't have the flexibility of
>> using different DIMMs and/or clocks?
> 
> It's a different world.  Again, thinking back to the PowerPC boards
> I've seen, they had "regular" DDR sockets.  Most ARM boards don't.
> You can design your board with whatever, and I know in prototyping
> folks make do swapping chips in and out (and if you look at the omap
> code, you can see where we have code to calculate the timing values
> and print them, or use provided hard-coded values).

Understood. We may have boards with DIMM slots. I do see value of a
fully functional DDR driver here.

> 
>>> The problem is that the DDR controller is usually
>>> vendor-specific. Perhaps the flip-side here is that there's not
>>> so much a generic DDR driver for mpc85xx but simply one vendor
>>> for mpc85xx.  Taking arch/powerpc/cpu/mpc85xx/ddr-gen3.c as what
>>> you're talking about, 
>>> arch/arm/cpu/armv7/omap-common/emif-common.c would be an
>>> ARM-world example (the 'EMIF' is found on a large variety of TI
>>> parts, not just "omap" ones).
> 
>> Does it make sense to share the Freescale DDR driver across ARM
>> and Powerpc? Or does it make more sense to selectively copy the
>> mpc8xxx DDR driver to Freescale ARM subfolder to start with. If the
>> similarity sustains then we merge them into common driver. If not,
>> we maintain two separated drivers.
> 
>> For those who is not familiar with, Freescale is extending products
>> to ARM cores. I am expecting peripherals stay relatively close, so
>> many driver can be reused.
> 
> I've been wondering when this would start to be visible.  If we are
> able to share the DDR controller code between mpc85xx and the ARM
> stuff you're talking about, we'll have to sort out someplace within
> drivers/ to place it, to avoid #include nightmares I suspect.  Other
> drivers should be easier to share at least :)
> 

I did a quick and dirty hack yesterday to move DDR driver to
drivers/ddr/fsl. I had to change many #include <asm/????>.

York