[U-Boot] [PATCH 00/35][RFC] arm: socfpga: Usability fixes

[Marek Vasut]

On Wednesday, September 17, 2014 at 12:29:54 AM, Dinh Nguyen wrote:

[...]

> >> Yes, tracked it down to get_ram_size(). I forced gd->ram_size to 1GB and
> >> it works fine for me now. I'll try to spend some cycles to debug the
> >> problem.
> > 
> > Hmmmmm, how much DRAM can the SoCFPGA chip drive in total ?
> 
> All of our devkits have at least 1 GiB and I have heard there is a
> variant with 2GiB in the wild. Spec says up to 4 GiB.

OK, I see. You cannot realistically map all those 4GiB into the 32-bit address 
space of an CortexA9, but on the other hand, all those bugs related to an CA9 
with 4GiB of DRAM should be fixed due to my work on Novena ;-)

> > Well, consider a theoretical SoCFPGA board with 128MiB of DRAM attached
> > to it, what happens if I try to write at address of the 129th MiB (which
> > is past the DRAM) ? Will this generate an DABT for the ARM core or will
> > some kind of DRAM "mirroring" or "wraparound" happen such that I would
> > write to the content of 1st MiB of the DRAM ?
> 
> We've encountered this issue downstream on a system with 1 GiB.

OK, so a wraparound happens ?

> > If I would get DABT, then there is a rather easy fix for that, see
> > arch/arm/cpu/arm926ejs/mxs/spl_mem_init.c and mxs_mem_get_size()
> > function. The function places an assembly return instruction into the
> > DABT handler entry position (offset 0x14 in ARM vector table IIRC) and
> > then runs the get_dram_size() . The assembly instruction only returns
> > from the DABT handler right past the code where the DABT happened. For
> > the get_ram_size(), the read from the unpopulated DRAM space contains
> > zeroes and the function doesn't even realize the DABT happened. But it
> > considers the DRAM invalid and thus this works correctly. That's how it
> > detects the amount of DRAM.
> > 
> > You might want to consider something similar if that's how it behaves on
> > SoCFPGA.
> 
> This could be the issue. I think Chin Liang would know about this more
> than me at this point. So I hope he can solve this quickly.

Sure, patch is welcome!

> Seems odd that the get_ram_size is working on your DENX board and not
> the devkit.

I _think_ I might have hacked this part up and it's still in the cleanup 
pipeline.

Best regards,
Marek Vasut