[PATCH v3] rockchip: board: Increase rng-seed size to make it sufficient for modern Linux

[Alex Shumsky]

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 vesrions 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://github.com/torvalds/linux/blob/7234e2ea0edd00bfb6bb2159e55878c19885ce68/drivers/char/random.c#L632
https://elixir.bootlin.com/linux/v5.18/source/drivers/char/random.c#L236
https://elixir.bootlin.com/linux/v5.19/source/drivers/char/random.c#L551

Signed-off-by: Alex Shumsky <alexthreed at gmail.com>
Fixes: d2048bafae40 ("rockchip: board: Add board_rng_seed() for all Rockchip devices")
---

Changes in v3:
- reword warning
- increase default rng_seed_size to support older linux as well
- reword commit message

Changes in v2:
- add env config knob rng_seed_size
- add warning for small rng_seed_size
- 12-character commit SHA in Fixes

 arch/arm/mach-rockchip/board.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/arm/mach-rockchip/board.c b/arch/arm/mach-rockchip/board.c
index 3fadf7e412..9ff1567fb1 100644
--- a/arch/arm/mach-rockchip/board.c
+++ b/arch/arm/mach-rockchip/board.c
@@ -472,9 +472,16 @@ __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'es
+		log_warning("Value for rng_seed_size too low (%lu) and likely insufficient for the Linux RNG initialization\n",
+			    len);
+	}
+
 	data = malloc(len);
 	if (!data) {
 		printf("Out of memory\n");
-- 
2.34.1