Broken support for 4GB DDR on 32-bit platforms

[Pali Rohár]

On Tuesday 17 May 2022 11:52:14 Tom Rini wrote:
> On Mon, May 16, 2022 at 11:56:51PM +0200, Pali Rohár wrote:
> > 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.
> 
> I'm not seeing the problem, sorry.  You run U-Boot in the normal 2GB

Ok, I will try to explain it again.

I want to run U-Boot in normal 2GB area. U-Boot normally store detected
RAM size into the gd->ram_size structure. And here is the issue.

32-bit unsigned C type cannot represent maximal 32-bit addressable size.
U-Boot tries to store 4GB value = 0x100000000 into gd->ram_size which
overflows to 0x00000000. And U-Boot then crashes because it expects that
can store some data at address "gd->ram_size - few_kb" as it expects
that RAM is in the area [0, gd->ram_size).

It is more clear what is the problem?

If I theoretically find 3.9GB RAM module (but such probably does not
exist) then U-Boot should work in normal 2GB area as number 3.9GB can be
stored into 32-bit unsigned type.

> area and tell Linux there's 3GB available.  I kinda recall now that yes,
> you can do a 3:1 split in Linux instead of 2:2 split, but in that case
> you still do 2:2 in U-Boot.  As you have seen configuring for anything
> else doesn't work.
> 
> -- 
> Tom