[U-Boot] [PATCH 4/6] ARM: tegra: Restrict usable RAM to 32-bit on 64-bit SoCs

[Thierry Reding]

From: Thierry Reding <treding at nvidia.com>

Most peripherals on Tegra can do DMA only to the lower 32-bit address
space, even on 64-bit SoCs. This limitation is typically overcome by the
use of an IOMMU. Since the IOMMU is not entirely trivial to set up and
serves no other purpose (I/O protection, ...) in U-Boot, restrict 64-bit
Tegra SoCs to the lower 32-bit address space for RAM. This ensures that
the physical addresses of buffers that are programmed into the various
DMA engines are valid and don't alias to lower addresses.

Cc: Tom Warren <twarren at nvidia.com>
Signed-off-by: Thierry Reding <treding at nvidia.com>
---
 board/nvidia/common/board.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/board/nvidia/common/board.c b/board/nvidia/common/board.c
index d945de456ea2..1134b31791db 100644
--- a/board/nvidia/common/board.c
+++ b/board/nvidia/common/board.c
@@ -264,3 +264,19 @@ void pad_init_mmc(struct mmc_host *host)
 #endif	/* T30 */
 }
 #endif	/* MMC */
+
+#ifdef CONFIG_ARM64
+/*
+ * Most hardware on 64-bit Tegra is still restricted to DMA to the lower
+ * 32-bits of the physical address space. Cap the maximum usable RAM area
+ * at 4 GiB to avoid DMA buffers from being allocated beyond the 32-bit
+ * boundary that most devices can address.
+ */
+ulong board_get_usable_ram_top(ulong total_size)
+{
+	if (gd->ram_top > 0x100000000)
+		return 0x100000000;
+
+	return gd->ram_top;
+}
+#endif
-- 
2.3.2