Broken support for 4GB DDR on 32-bit platforms

[Pali Rohár]

On Monday 16 May 2022 08:31:43 Tom Rini wrote:
> On Sat, May 14, 2022 at 01:00:06AM +0200, Pali Rohár wrote:
> 
> > Hello! I tried to enable support for 2GB+ of DDR memory (with 4GB DDR3)
> > on powerpc P2020 board in 32-bit addressing mode and U-Boot crashed
> > during startup.
> > 
> > I figured out that issue is not powerpc specific, but rather generic to
> > all 32-bit platforms. U-Boot stores memory size into phys_size_t type
> > (gd->ram_size) and last mapped memory address increased by one byte into
> > phys_addr_t type (gd->ram_top).
> > 
> > Despite size 4GB fits into 32-bit addressing mode, it does not fit into
> > above two variables, and it overflows to zero. U-Boot then see zero RAM
> > size and crashes.
> > 
> > I tried to workaround this issue by changing both phys_size_t and
> > phys_addr_t types to 64-bit. But it did not helped because U-Boot on
> > many places cast gd->ram_size or gd->ram_top to ulong type, which is
> > 32-bit on 32-bit platforms.
> > 
> > Next I changed ulong parameters of board_get_usable_ram_top() function
> > to u64.
> > 
> > This still was not enough because config value CONFIG_MAX_MEM_MAPPED is
> > ignored on one important place -- in function get_effective_memsize().
> > This config value takes effect only when also CONFIG_VERY_BIG_RAM is
> > set.
> > 
> > Finally With this change I was able to start U-Boot with more than 2GB
> > of DDR memory inserted in SODIMM slot on P2020.
> > 
> > How to fix issues with gd->ram_size and gd->ram_top? That +1 byte is
> > really stupid limitation. Changing phys_size_t and phys_addr_t types
> > unconditionally to 64-bit? Or something else?
> > 
> > And what is the purpose of CONFIG_VERY_BIG_RAM config option? Why is
> > CONFIG_MAX_MEM_MAPPED check skipped in get_effective_memsize() function,
> > but is not skipped on many more places?
> 
> So, there's two parts to this, as I recall it all.  First, even on 64bit
> platforms we contain ourselves to 32bit address space and even then
> something within the "old" 2GB window.  We then set a CONFIG option to
> not mess with the memory node in DT which has the real value.  Second,
> for 32bit platforms which can support 4GB memory, or more, some further
> games need to be played, typically I believe around initializing the
> memory controller (I'm more confident of that for dra7xx_evm, which I
> don't have the big memory version of, just a small memory one) so that
> Linux can do whatever needs doing to enable "36bit" typically address
> support.  Looking at the other P*36BIT* configs might give you some more
> clues about what to do on your platform, or at least who might still be
> able to explain and test things on the PowerPC side.

I know about 36-bit addressing on e500v2 but I'm not going to enable it
due to performance reasons (see Freescale AN4064 [1]). So I want to
stick with 32-bit addressing for 2GB+ memory usage (around 3GB; it is
4GB minus memory used by peripherals; which is still more than 2GB).

And due to 32-bit type for phys_size_t, phys_addr_t and casting these
types to ulong is an issue. Plus issue with CONFIG_VERY_BIG_RAM and
CONFIG_MAX_MEM_MAPPED as I already wrote.

[1] - https://www.nxp.com/docs/en/application-note/AN4064.pdf