[RFC] Raspberry Pi: copy /system/linux, {serial, revision} from firmware

[Fiona Klute]

Both the Raspberry Pi downstream kernel [1] and some userspace tools
[2] use the revision property to identify the board, and the latter
fails if it is absent. The firmware creates the /system node, so we
need to do the same.

[1] https://github.com/raspberrypi/linux/blob/0f292fbb6346b05766152902076895558ac23f9a/arch/arm/mach-bcm/board_bcm2835.c#L23-L33
[2] https://github.com/jgarff/rpi_ws281x/blob/7fc0bf8b31d715bbecf28e852ede5aaa388180da/rpihw.c#L579

Signed-off-by: Fiona Klute <fiona.klute at gmx.de>
Cc: Matthias Brugger <mbrugger at suse.com>
Cc: Peter Robinson <pbrobinson at gmail.com>
Cc: Tom Rini <trini at konsulko.com>
---
I'm sending this as RFC because I'm not sure if creating a temporary
overlay is the best approach here, advice on a better way to create the
/system node if needed is very much welcome. I know I'll need to add
more error checks, I'll do that after I know I have the right approach.

 board/raspberrypi/rpi/rpi.c | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/board/raspberrypi/rpi/rpi.c b/board/raspberrypi/rpi/rpi.c
index aa39afa338a..647716127c7 100644
--- a/board/raspberrypi/rpi/rpi.c
+++ b/board/raspberrypi/rpi/rpi.c
@@ -550,6 +550,41 @@ int copy_property(void *dst, void *src, char *path, char *property)
 	return fdt_setprop(dst, dst_offset, property, prop, len);
 }
 
+int copy_system_node(void *fdt, void *fw_fdt)
+{
+	const fdt32_t *prop;
+	char fdto[2048];
+	int src_offset;
+	int len;
+
+	src_offset = fdt_path_offset(fw_fdt, "/system");
+	if (src_offset < 0)
+		return -1;
+
+	fdt_create(fdto, sizeof(fdto));
+	fdt_finish_reservemap(fdto);
+	fdt_begin_node(fdto, "");
+	fdt_begin_node(fdto, "fragment");
+	fdt_property_string(fdto, "target-path", "/");
+	fdt_begin_node(fdto, "__overlay__");
+	fdt_begin_node(fdto, "system");
+
+	prop = fdt_getprop(fw_fdt, src_offset, "linux,serial", &len);
+	if (prop)
+		fdt_property(fdto, "linux,serial", prop, len);
+	prop = fdt_getprop(fw_fdt, src_offset, "linux,revision", &len);
+	if (prop)
+		fdt_property(fdto, "linux,revision", prop, len);
+
+	fdt_end_node(fdto);
+	fdt_end_node(fdto);
+	fdt_end_node(fdto);
+	fdt_end_node(fdto);
+	fdt_finish(fdto);
+
+	fdt_overlay_apply_verbose(fdt, fdto);
+}
+
 /* Copy tweaks from the firmware dtb to the loaded dtb */
 void  update_fdt_from_fw(void *fdt, void *fw_fdt)
 {
@@ -560,6 +595,8 @@ void  update_fdt_from_fw(void *fdt, void *fw_fdt)
 	/* The firmware provides a more precise model; so copy that */
 	copy_property(fdt, fw_fdt, "/", "model");
 
+	copy_system_node(fdt, fw_fdt);
+
 	/* memory reserve as suggested by the firmware */
 	copy_property(fdt, fw_fdt, "/", "memreserve");
 
-- 
2.47.2