[PATCH] rpi: bcm2712: Extend DRAM mapping to 16 GiB

[Peter Robinson]

On Mon, 11 May 2026 at 12:40, Peter Robinson <pbrobinson at gmail.com> wrote:
>
> Hi Simon,
>
> Thanks for this.
>
> On Sat, 9 May 2026 at 00:33, Simon Glass <sjg at chromium.org> wrote:
> >
> > The bcm2712 memory map only covers the first 1 GiB of DRAM, but Pi 5
> > models ship with up to 16 GiB. Accesses above 1 GiB fault because the
> > range is not present in the MMU page tables.
> >
> > Allocations above 1 GiB used to be blocked at the lmb layer because
> > memory above ram_top was reserved as no-overwrite. Commit 0efb6bc3e36d
> > ("lmb: Reinstate access to memory above ram_top") removed that
> > reservation, so callers like tftp now route allocations into the
> > unmapped range and trigger a fault.
> >
> > Extend the first DRAM entry to 16 GiB to cover the largest addressable
> > range for any production Pi 5 model. Smaller models are unaffected
> > because aarch64 only faults on actual access of unbacked memory, and
> > no peripheral region lies between 1 GiB and 64 GiB on this SoC
> > (peripherals start at 0x1000000000).
>
> So a couple of questions:
> 1) Do we need to do this for the Pi4 (bcm2711) too? It's currently at ~4Gb.
> 2) On the bcm2711 it sits at just under 4gb, initially that was
> 4Gb-16Mb AFAICT (917a1e9a78fe) and then it was reduced in a followup
> commit (c44b3f523c48) because of overlap with "device's IO area"
> 3) why doesn't the rpi5 have the overlap issues?

Also at least on a 1Gb RPi5 it's still crashing for me.

> Peter
>
> > Fixes: 0efb6bc3e36d ("lmb: Reinstate access to memory above ram_top")
> > Signed-off-by: Simon Glass <sjg at chromium.org>
> > ---
> >
> >  arch/arm/mach-bcm283x/init.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/arm/mach-bcm283x/init.c b/arch/arm/mach-bcm283x/init.c
> > index 7a1de22e0ae..3d11c9e5c3b 100644
> > --- a/arch/arm/mach-bcm283x/init.c
> > +++ b/arch/arm/mach-bcm283x/init.c
> > @@ -69,10 +69,10 @@ static struct mm_region bcm2711_mem_map[MEM_MAP_MAX_ENTRIES] = {
> >
> >  static struct mm_region bcm2712_mem_map[MEM_MAP_MAX_ENTRIES] = {
> >         {
> > -               /* First 1GB of DRAM */
> > +               /* DRAM, up to the 16 GiB maximum supported by Pi 5 models */
> >                 .virt = 0x00000000UL,
> >                 .phys = 0x00000000UL,
> > -               .size = 0x40000000UL,
> > +               .size = 0x400000000UL,
> >                 .attrs = PTE_BLOCK_MEMTYPE(MT_NORMAL) |
> >                          PTE_BLOCK_INNER_SHARE
> >         }, {
> > ---
> > base-commit: 3cdd19089f1b1b7cd08530f33ff4708abcfd426c
> > branch: rpi-us
> >
> > --
> > 2.43.0
> >