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

[Peter Robinson]

On Wed, 3 Jun 2026 at 12:12, Marek Vasut <marek.vasut at mailbox.org> wrote:
>
> On 6/3/26 1:02 PM, Peter Robinson wrote:
> > On Sat, 30 May 2026 at 16:06, 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
> >>> +     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
> >>> diff --git a/lib/lmb.c b/lib/lmb.c
> >>> index 8f12c6ad8e5..a8a988eedc0 100644
> >>> --- a/lib/lmb.c
> >>> +++ b/lib/lmb.c
> >>> @@ -611,6 +611,7 @@ static __maybe_unused void lmb_reserve_common_spl(void)
> >>>    static void lmb_add_memory(void)
> >>>    {
> >>>        int i;
> >>> +     phys_addr_t bank_end;
> >>>        phys_size_t size;
> >>>        u64 ram_top = gd->ram_top;
> >>>        struct bd_info *bd = gd->bd;
> >>> @@ -625,8 +626,25 @@ static void lmb_add_memory(void)
> >>>        for (i = 0; i < CONFIG_NR_DRAM_BANKS; i++) {
> >>>                size = bd->bi_dram[i].size;
> >>>
> >>> -             if (size)
> >>> +             if (size) {
> >>>                        lmb_add(bd->bi_dram[i].start, size);
> >>> +                     if (IS_ENABLED(CONFIG_LMB_LIMIT_DMA_BELOW_4G))
> >>
> >> This conditional should be inverted.
> >>
> >> Peter Robinson , can you test this change on RPi ?
> >
> > That doesn't fix the problem for me on a RPi4 4GB or a RPi5 8Gb.
> Did you invert the if (IS_ENABLED...) conditional ?
>
> Was CONFIG_LMB_LIMIT_DMA_BELOW_4G enabled in your config (maybe the
> Kconfig entry "default y" part needs to be tweaked to cover these devices?)
>
> If both conditions are met, that should behave like the old code.

I dropped my revert and applied the patch, checked the new config
option was in the generated .config (I mean you could test that with a
"make rpi_arm64_defconfig" and checking the output).

Peter