[PATCH v5 2/3] fdt: add support for rng-seed

[Hsin-Yi Wang]

Introducing a chosen node, rng-seed, which is an entropy that can be
passed to kernel called very early to increase initial device
randomness. Bootloader should provide this entropy and the value is
read from /chosen/rng-seed in DT.

Signed-off-by: Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx>
---
change log v4->v5:
* use fdt_nop_property() since property might not be wiped entirely if
following entries are smaller than property length. 
---
 Documentation/devicetree/bindings/chosen.txt | 14 ++++++++++++++
 drivers/of/fdt.c                             | 10 ++++++++++
 2 files changed, 24 insertions(+)

diff --git a/Documentation/devicetree/bindings/chosen.txt b/Documentation/devicetree/bindings/chosen.txt
index 45e79172a646..678e81bc4383 100644
--- a/Documentation/devicetree/bindings/chosen.txt
+++ b/Documentation/devicetree/bindings/chosen.txt
@@ -28,6 +28,20 @@ mode) when EFI_RNG_PROTOCOL is supported, it will be overwritten by
 the Linux EFI stub (which will populate the property itself, using
 EFI_RNG_PROTOCOL).
 
+rng-seed
+-----------
+
+This property serves as an entropy to add device randomness. It is parsed
+as a byte array, e.g.
+
+/ {
+	chosen {
+		rng-seed = <0x31 0x95 0x1b 0x3c 0xc9 0xfa 0xb3 ...>;
+	};
+};
+
+This random value should be provided by bootloader.
+
 stdout-path
 -----------
 
diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
index de893c9616a1..9d63330582bb 100644
--- a/drivers/of/fdt.c
+++ b/drivers/of/fdt.c
@@ -24,6 +24,7 @@
 #include <linux/debugfs.h>
 #include <linux/serial_core.h>
 #include <linux/sysfs.h>
+#include <linux/random.h>
 
 #include <asm/setup.h>  /* for COMMAND_LINE_SIZE */
 #include <asm/page.h>
@@ -1079,6 +1080,7 @@ int __init early_init_dt_scan_chosen(unsigned long node, const char *uname,
 {
 	int l;
 	const char *p;
+	const void *rng_seed;
 
 	pr_debug("search \"chosen\", depth: %d, uname: %s\n", depth, uname);
 
@@ -1113,6 +1115,14 @@ int __init early_init_dt_scan_chosen(unsigned long node, const char *uname,
 
 	pr_debug("Command line is: %s\n", (char*)data);
 
+	rng_seed = of_get_flat_dt_prop(node, "rng-seed", &l);
+	if (rng_seed && l > 0) {
+		add_device_randomness(rng_seed, l);
+
+		/* try to clear seed so it won't be found. */
+		fdt_nop_property(initial_boot_params, node, "rng-seed");
+	}
+
 	/* break now */
 	return 1;
 }
-- 
2.20.1