[PATCH] arm64: dts: rockchip: Describe why is HWRNG disabled in RK356x base dtsi

[Dragan Simic]

Despite the presence of the hardware random number generator (HWRNG) in the
different Rockchip RK356x SoC variants, it remains disabled for the RK3566
SoC because testing showed [1] that it produces unacceptably low quality of
random data, for some yet unknown reason.  The HWRNG is enabled for the RK3568
SoC, on which the testing showed good quality of the generated random data.

To avoid possible confusion in the future, [2] let's have this described
briefly in the RK356x base SoC dtsi.

[1] https://lore.kernel.org/linux-rockchip/cover.1720969799.git.daniel@makrotopia.org/T/#u
[2] https://lore.kernel.org/linux-rockchip/20241201234613.52322-1-pbrobinson@gmail.com/T/#u

Signed-off-by: Dragan Simic <dsimic at manjaro.org>
---
 arch/arm64/boot/dts/rockchip/rk356x-base.dtsi | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/arm64/boot/dts/rockchip/rk356x-base.dtsi b/arch/arm64/boot/dts/rockchip/rk356x-base.dtsi
index 62be06f3b863..ab8f42c0a843 100644
--- a/arch/arm64/boot/dts/rockchip/rk356x-base.dtsi
+++ b/arch/arm64/boot/dts/rockchip/rk356x-base.dtsi
@@ -1032,6 +1032,11 @@ sdhci: mmc at fe310000 {
 		status = "disabled";
 	};
 
+	/*
+	 * Testing showed that the HWRNG found in RK3566 produces unacceptably
+	 * low quality of random data, so the HWRNG isn't enabled for all RK356x
+	 * SoC variants despite its presence.
+	 */
 	rng: rng at fe388000 {
 		compatible = "rockchip,rk3568-rng";
 		reg = <0x0 0xfe388000 0x0 0x4000>;