rockchip: board: Increase rng-seed size to make it sufficient for modern Linux

Increase rng-seed size to make Linux happy and initialize rng pool instantly.
Linux 5.19+ requires 32 bytes of entropy to initialize random pool, but u-boot
currently provides only 8 bytes.
Linux 5.18 and probably some versions before it used to require 64 bytes.
Bump min value to 64 bytes to be on a safe side.

Boot with 8 byte rng-seed (Linux 6.11):
    # dmesg | grep crng
    [   12.089286] random: crng init done
Boot with 32 byte rng-seed (Linux 6.11):
    # dmesg | grep crng
    [    0.000000] random: crng init done

Linux source references:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/char/random.c?h=v5.19#n551
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/char/random.c?h=v5.18#n236

Signed-off-by: Alex Shumsky <alexthreed@gmail.com>
Fixes: d2048bafae ("rockchip: board: Add board_rng_seed() for all Rockchip devices")
Reviewed-by: Dragan Simic <dsimic@manjaro.org>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>

arch/arm/mach-rockchip/board.c

@@ -472,9 +472,18 @@ __weak int misc_init_r(void)
 __weak int board_rng_seed(struct abuf *buf)
 {
 	struct udevice *dev;
-	size_t len = 0x8;
+	ulong len = env_get_ulong("rng_seed_size", 10, 64);
 	u64 *data;
 
+	if (len < 64) {
+		/*
+		 * rng_seed_size should be at least 32 bytes for Linux 5.19+,
+		 * or 64 for older Linux kernel versions
+		 */
+		log_warning("Value for rng_seed_size (%lu) too low, Linux kernel RNG may fail to initialize early\n",
+			    len);
+	}
+
 	data = malloc(len);
 	if (!data) {
 		printf("Out of memory\n");

common/Kconfig

@@ -928,6 +928,9 @@ config BOARD_RNG_SEED
 	  new seed for use on subsequent boots, and whether or not the
 	  kernel should account any entropy from the given seed.
 
+	  Default seed size (64 bytes) can be overridden by a decimal
+	  environment variable rng_seed_size.
+
 endmenu
 
 menu "Update support"

doc/usage/environment.rst

@@ -323,6 +323,11 @@ netretry
     Useful on scripts which control the retry operation
     themselves.
 
+rng_seed_size
+    Size of random value added to device-tree node /chosen/rng-seed.
+    This variable is given as a decimal number.
+    If unset, 64 bytes is used as the default.
+
 silent_linux
     If set then Linux will be told to boot silently, by
     adding 'console=' to its command line. If "yes" it will be

include/fdt_support.h

@@ -202,7 +202,8 @@ int ft_board_setup(void *blob, struct bd_info *bd);
  *
  * This function is called if CONFIG_BOARD_RNG_SEED is set, and must
  * be provided by the board. It should return, via @buf, some suitable
- * seed value to pass to the kernel.
+ * seed value to pass to the kernel. Seed size could be set in a decimal
+ * environment variable rng_seed_size and it defaults to 64 bytes.
  *
  * @param buf         A struct abuf for returning the seed and its size.
  * @return            0 if ok, negative on error.