[PATCH] lmb: Reinstate access to memory above ram_top

[Heinrich Schuchardt]

Am 28. Januar 2026 00:48:40 MEZ schrieb Marek Vasut <marek.vasut+renesas at mailbox.org>:
>Revert commit eb052cbb896f ("lmb: add and reserve memory above ram_top")
>and commit 1a48b0be93d4 ("lmb: prohibit allocations above ram_top even from
>same bank"). These are based on incorrect premise of the first commit, that
>"U-Boot does not use memory above ram_top". While U-Boot itself indeed does
>not and should not use memory above ram_top, user can perfectly well use
>that memory from the U-Boot shell, for example to load content in there.
>
>Currently, attempt to use that memory to load large image using TFTP ends
>with "TFTP error: trying to overwrite reserved memory...". With this change
>in place, the memory can be used again.

IIRC there are drivers that have issues with memory above 4 GiB and boards that artificially set ram_top to 4 GiB to work around these issues. We need to carefully consider why the reverted patch was introduced.

Why does your board use a 4GiB ram_top and not place ram_top at the physical top?

Best regards

Heinrich

>
>Fixes: eb052cbb896f ("lmb: add and reserve memory above ram_top")
>Fixes: 1a48b0be93d4 ("lmb: prohibit allocations above ram_top even from same bank")
>Reported-by: Yuya Hamamachi <yuya.hamamachi.sx at renesas.com>
>Signed-off-by: Marek Vasut <marek.vasut+renesas at mailbox.org>
>---
>Cc: Andrew Goodbody <andrew.goodbody at linaro.org>
>Cc: Casey Connolly <casey.connolly at linaro.org>
>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: Jonas Karlman <jonas at kwiboo.se>,
>Cc: Mark Kettenis <kettenis at openbsd.org>
>Cc: Sam Protsenko <semen.protsenko at linaro.org>
>Cc: Sughosh Ganu <sughosh.ganu at linaro.org>
>Cc: Tom Rini <trini at konsulko.com>
>Cc: u-boot at lists.denx.de
>---
> lib/lmb.c | 17 +----------------
> 1 file changed, 1 insertion(+), 16 deletions(-)
>
>diff --git a/lib/lmb.c b/lib/lmb.c
>index e2d9fe86c14..8f12c6ad8e5 100644
>--- a/lib/lmb.c
>+++ b/lib/lmb.c
>@@ -611,7 +611,6 @@ 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;
>@@ -625,23 +624,9 @@ static void lmb_add_memory(void)
> 
> 	for (i = 0; i < CONFIG_NR_DRAM_BANKS; i++) {
> 		size = bd->bi_dram[i].size;
>-		bank_end = bd->bi_dram[i].start + size;
> 
>-		if (size) {
>+		if (size)
> 			lmb_add(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);
>-		}
> 	}
> }
>