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

[Marek Vasut]

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 ?

> +				continue;
> +
> +			bank_end = bd->bi_dram[i].start + size;
> +
> +			/*
> +			 * Reserve memory above ram_top as
> +			 * no-overwrite so that it cannot be
> +			 * allocated
> +			 */
> +			if (bd->bi_dram[i].start >= ram_top)
> +				lmb_reserve(bd->bi_dram[i].start, size,
> +					    LMB_NOOVERWRITE);
> +			else if (bank_end > ram_top)
> +				lmb_reserve(ram_top, bank_end - ram_top,
> +					    LMB_NOOVERWRITE);
> +		}
>   	}
>   }
>   

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