linux-kernel - Re: [PATCH] Add Ingenic JZ4780 hardware RNG driver

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Date:   Fri, 19 Aug 2016 18:24:12 +0530
From:   PrasannaKumar Muralidharan <prasannatsmkumar@...il.com>
To:     noloader@...il.com
Cc:     linux-crypto@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mips@...ux-mips.org
Subject: Re: [PATCH] Add Ingenic JZ4780 hardware RNG driver

> 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?