Hi Marcelo,
On 26/7/26 16:41, Marcelo Manzo wrote:
raspi4_modify_dtb() decides whether to add a second memory node above
the 1 GiB peripheral hole by checking info->ram_size -- but that field
is the boot loader's RAM budget for loading the kernel/initrd/dtb
image, itself always capped to at most UPPER_RAM_BASE - vcram_size by
raspi_base_machine_init(). Since that capped value can never exceed
UPPER_RAM_BASE by construction, the condition was never true for any
raspi4b configuration, and the second node was never added: the guest
never saw more than ~1 GiB of its nominal RAM, regardless of the
machine's actual size.
board_ram_size(info->board_id), computed one line above in the same
function, is the value that was actually needed -- the board's real
total RAM, not the boot loader's own budget for where it's allowed to
place the kernel image.
Confirmed via direct measurement inside the guest ("free -h" /
/proc/meminfo) on raspi4b's default 2 GiB configuration, before and
after:
before: MemTotal: 943524 kB (~921 MiB)
after: MemTotal: 1905824 kB (~1861 MiB)
Also verified against two real, unmodified Raspberry Pi OS releases
(Debian 11/Bullseye and Debian 13/Trixie): both now report ~1.8 GiB of
usable RAM instead of ~900 MiB, with clean boots, working SSH, and no
kernel errors on either.
Signed-off-by: Marcelo Manzo <[email protected]>
---
hw/arm/raspi4b.c | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)
diff --git a/hw/arm/raspi4b.c b/hw/arm/raspi4b.c
index b92840e1b6..ca246ebb35 100644
--- a/hw/arm/raspi4b.c
+++ b/hw/arm/raspi4b.c
@@ -64,7 +64,19 @@ static void raspi4_modify_dtb(const struct arm_boot_info
*info, void *fdt)
ram_size = board_ram_size(info->board_id);
- if (info->ram_size > UPPER_RAM_BASE) {
+ /*
+ * Bug: this used to compare info->ram_size (the boot-loader's RAM
+ * budget for loading the kernel/initrd/dtb, itself capped to at most
+ * UPPER_RAM_BASE - vcram_size by raspi_base_machine_init()) rather
+ * than the board's actual total RAM computed just above. Since that
+ * capped value can never exceed UPPER_RAM_BASE by construction, this
+ * condition was never true for any raspi4b configuration -- the
+ * second memory node was never added, and the guest never saw more
+ * than ~1 GiB regardless of the machine's nominal RAM size. Confirmed
+ * via direct measurement: default -m 2G returns ~916 MiB from
+ * "free -h" inside the guest, not 2 GiB.
+ */
No need to add this comment in the code, we already have the git
history. With the comment removed:
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
+ if (ram_size > UPPER_RAM_BASE) {
raspi_add_memory_node(fdt, UPPER_RAM_BASE, ram_size - UPPER_RAM_BASE);
}
}
