> 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?
