Please forgive my ignorance Prasanna...

For the JZ4780 I have, there are two registers in play. The first is
the control register which enables/disables the RNG. The control
register is named ERNG. The second register is the data register, and
it produces the random stream. The data register is named RNG. ERNG is
located at 0x100000D8 and RNG is located at 0x100000DC. This kind of
confuses me because I don't see where 0x100000D8 is ever added to
those values (maybe its in the descriptor?):

+#define REG_RNG_CTRL   0x0
+#define REG_RNG_DATA   0x4

The base address 0x100000D8 is defined in jz4780.dtsi file.
REG_RNG_CTRL and REG_RNG_DATA are offsets. In jz4780_rng_readl and
jz4780_rng_writel functions the ioremap'd base address is added with
offset.

Also, testing with a userland PoC for the device, you have to throttle
reads from RNG register. If reads occur with a 0 delay, then the
random value appears fixed. If the delay is too small, then you can
watch random values being shifted-in in a barrel like fashion.
Unfortunately, the manual did not discuss how long to wait for a value
to be ready. I found spinning in a loop for 5000 was too small and
witnessed the shifting; while spinning in a loop for 10000 avoided the
shift observation. I don't what number of JIFFIES that translates to.

I can calculate the speed and make sure the delay is met in the
driver. Thanks a lot for providing this info.

Finally, from looking at the native Ingenic driver (which was not very
impressive), they enabled/disabled the RNG register on demand. There
was also a [possible related] note in the manual about not applying
VCC for over a second. I can only say "possibly related" because I was
not sure if the register was part of the controller they were
discussing. The userland PoC worked fine when enabling/disabling the
RNG register. So I'm not sure about this (from jz4780_rng_probe):

+       platform_set_drvdata(pdev, jz4780_rng);
+       jz4780_rng_writel(jz4780_rng, 1, REG_RNG_CTRL);
+       ret = hwrng_register(&jz4780_rng->rng);

And this (from jz4780_rng_remove):

+       jz4780_rng_writel(jz4780_rng, 0, REG_RNG_CTRL);
+       hwrng_unregister(&jz4780_rng->rng);

Anyway, I hope that helps you avoid some land mines (if they are present).

Looking at JZ4780 Programmers manual I could not find any info about
VCC. Can you point me to it?
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