[U-Boot] [PATCH V2 13/14] ARM: AM43xx: GP_EVM: Add support for DDR3

[Vaibhav Bedia]

On Wed, Nov 27, 2013 at 4:34 AM, Lokesh Vutla <lokeshvutla at ti.com> wrote:
[...]
> Ideally the default value should be exported from e-fuse values.
> EMIF does some HW sequence according to the value exported here. This filed tells
> what type of memory it is.
>

No, eFuse is not the right place for this information. If you were to do that
you would be restricting what memory type is supported on a particular
device that gets shipped and that's not a scalable model.

> I understand the point what if efuse is not blown. I am using this only after we write into sdarm_config register.
> This can confirm that we get a correct value.
>  If this is not sufficient we can hardcode the register during startup only ?

I don't see any value in writing some value in the EMIF register at one place
and then using that here. If you already know what register values to program
you already know the memory type.

Another place where the memory type information is needed is when the device
enters low power state and the EMIF contents are lost. Rather than having to
invent one more way of detecting the memory type in resume from low power state
you could work out a persistent storage area (RTC scratchpad maybe? assuming
the RTC is powered up) and use that in all the places. Requires some thought
about the whole sequence but definitely doable IMHO.

> One more thing is we can get from MR registers of DDR.
> But for DDR3 we cannot access MR registers. That is why I didn't go with this approach.
>

So now you know why things just worked on OMAP3 (and OMAP4?) and not
on TI81xx, AM335x and AM437x :)

> Currently EEPROM doesn't have any details about DDR.
> Please let me know if this approach is not good enough.
>

Since there's no speced way of detecting the memory type you should push for
the right information to be made available via the EEPROM and write
generic code.
But hey that's for you to decide :)

Regards,
Vaibhav