[PATCH] [RFC] lmb: Optionally limit available memory to 4 GiB on limited systems

[Ilias Apalodimas]

Hi Marek,

On Sat, 30 May 2026 at 18:06, Marek Vasut <marek.vasut at mailbox.org> wrote:
>
> On 5/30/26 4:45 PM, Marek Vasut wrote:
> > Some architectures can not DMA above 4 GiB boundary,
> > limit available memory to memory below 4 GiB boundary.
> >
> > Signed-off-by: Marek Vasut <marek.vasut+renesas at mailbox.org>
> > ---
> > Cc: Andrew Goodbody <andrew.goodbody at linaro.org>
> > Cc: David Lechner <dlechner at baylibre.com>
> > Cc: Heiko Schocher <hs at nabladev.com>
> > Cc: Heinrich Schuchardt <xypron.glpk at gmx.de>
> > Cc: Holger Brunck <holger.brunck at hitachienergy.com>
> > Cc: Ilias Apalodimas <ilias.apalodimas at linaro.org>
> > Cc: Peter Robinson <pbrobinson at gmail.com>
> > Cc: Quentin Schulz <quentin.schulz at cherry.de>
> > Cc: Simon Glass <sjg at chromium.org>
> > Cc: Sughosh Ganu <sughosh.ganu at arm.com>
> > Cc: Tom Rini <trini at konsulko.com>
> > Cc: u-boot at lists.denx.de
> > ---
> >   lib/Kconfig |  8 ++++++++
> >   lib/lmb.c   | 20 +++++++++++++++++++-
> >   2 files changed, 27 insertions(+), 1 deletion(-)
> >
> > diff --git a/lib/Kconfig b/lib/Kconfig
> > index 77ebc79e1db..29911068a69 100644
> > --- a/lib/Kconfig
> > +++ b/lib/Kconfig
> > @@ -1295,6 +1295,14 @@ config SPL_LMB_ARCH_MEM_MAP
> >         memory map. Enable this config in such scenarios which allow
> >         architectures and boards to define their own memory map.
> >
> > +config LMB_LIMIT_DMA_BELOW_4G

This seems the right idea for now, but is it really 4G or ram_top?

> > +     bool
> > +     depends on LMB
> > +     default y if ARCH_BCM283X
> > +     help
> > +       Some architectures can not DMA above 4 GiB boundary,
> > +       limit available memory to memory below 4 GiB boundary.
> > +
> >   config PHANDLE_CHECK_SEQ
> >       bool "Enable phandle check while getting sequence number"
> >       help
[...]

Cheers
/Ilias