Issues with HW RNG on Exynos 5422

[Heiner Kallweit <hkallweit1@gmail.com>]

I'm having issues making the hardware RNG work on a Samsung Exynos 5422 (Odroid XU4) with kernel 4.2rc6.
No random number generation is started if I write the appropriate value (0x18) to the hash control register.

What I did so far:
Splitted the sss DT node in exynos5420.dtsi into one for the s5p-sss driver and one for the exynos-rng driver.
(s5p-sss doesn't seem to need the hash registers from offset 0x400)

sss: sss@10830000 {
	icompatible = "samsung,exynos4210-secss";
	reg = <0x10830000 0x400>;
	interrupts = <0 112 0>;
	clocks = <&clock CLK_SSS>;
	clock-names = "secss";
};

rng: rng@10830400 {
	compatible = "samsung,exynosrng-secss";
	reg = <0x10830400 0x300>;
	clocks = <&clock CLK_SSS>;
	clock-names = "secss";
};

The DT binding is just for testing and after adding some DT glue logic (of_device_id table) to the exynos-rng driver
it binds to the rng platform device.
The clock also seems to be ok with a rate of 266 MHz.
As is the driver hangs in a loop because the PRNG_DONE in the status register bit never gets set.

I traced it back to the hash control register not accepting value 0x8 (or 0x18 incl. the start bit) for the PRNG.
Writing a value and reading it back works for values from 0 to 5 only.
As I have no SSS datasheet my only other reference is drivers/crypto/ace_sha.h in the uboot source code
which also uses the HW RNG.

Any hint would be appreciated.