Re: [PATCH 1/2] hwrng: Add support for ASPEED RNG

[linux]

Hi Joel,

On 2020-01-20 19:53, Joel Stanley wrote:
> Hi,
>
> On Mon, 20 Jan 2020 at 15:12, Oscar A Perez <linux@xxxxxxxxxxxxxxx> 
> wrote:
> > This minimal driver adds support for the Hardware Random Number 
> > Generator
> > that comes with the AST2400/AST2500/AST2600 SOCs from AspeedTech.
> >
> > The HRNG on these SOCs uses Ring Oscillators working together to 
> > generate
> > a stream of random bits that can be read by the platform via a 32bit 
> > data
> > register.
>
> Thanks for the patch.
>
> We've been using the timeriomem-rng driver for the past few years on
> aspeed hardware. You can see how that's set up by looking at
> arch/arm/boot/dts/aspeed-g{4,5,6}.dtsi
>
> I suggest we continue to use the generic driver.
>
> Cheers,
>
> Joel

Thanks for reviewing the patch.

The RNG on Aspeed hardware allows eight different modes for combining 
its four internal Ring Oscillators that together generate a stream of 
random bits. However, the timeriomem-rng driver does not allow for mode 
selection so, the Aspeed RNG with this generic driver runs always on 
mode 'seven' (The default value for mode according to the AspeedTech 
datasheets).

I've performed some testings on this Aspeed RNG using the NIST 
Statistical Test Suite (NIST 800-22r1a) and, the results I got show that 
the default mode 'seven' isn't producing the best entropy and linear 
rank when compared against the other modes available on these SOCs.  On 
the other hand, the driver that I'm proposing here allows for mode 
selection which would help improve the random output for those looking 
to get the best out of this Aspeed RNG.

Thanks and regards,

Oscar A Perez

> > Signed-off-by: Oscar A Perez <linux@xxxxxxxxxxxxxxx>
> > ---
> >  .../devicetree/bindings/rng/aspeed-rng.yaml   | 90 
> > +++++++++++++++++++
> >  1 file changed, 90 insertions(+)
> >  create mode 100644 
> > Documentation/devicetree/bindings/rng/aspeed-rng.yaml
> >
> > diff --git a/Documentation/devicetree/bindings/rng/aspeed-rng.yaml 
> > b/Documentation/devicetree/bindings/rng/aspeed-rng.yaml
> > new file mode 100644
> > index 000000000000..06070ebe1c33
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/rng/aspeed-rng.yaml
> > @@ -0,0 +1,90 @@
> > +# SPDX-License-Identifier: GPL-2.0
> > +%YAML 1.2
> > +---
> > +$id: "http://devicetree.org/schemas/rng/aspeed-rng.yaml#";
> > +$schema: "http://devicetree.org/meta-schemas/core.yaml#";
> > +
> > +
> > +title: Bindings for Aspeed Hardware Random Number Generator
> > +
> > +
> > +maintainers:
> > +  - Oscar A Perez <linux@xxxxxxxxxxxxxxx>
> > +
> > +
> > +description: |
> > +  The HRNG on the AST2400/AST2500/AST2600 SOCs from AspeedTech  uses 
> > four Ring
> > +  Oscillators working together to generate a stream of random bits 
> > that can be
> > +  read by the platform via a 32bit data register every one 
> > microsecond.
> > +  All the platform has to do is to provide to the driver the 
> > 'quality' entropy
> > +  value, the  'mode' in which the combining  ROs will generate the  
> > stream  of
> > +  random bits and, the 'period' value that is used as a wait-time 
> > between reads
> > +  from the 32bit data register.
> > +
> > +
> > +properties:
> > +  compatible:
> > +    oneOf:
> > +      - items:
> > +          - enum:
> > +              - aspeed,ast2400-rng
> > +              - aspeed,ast2500-rng
> > +              - aspeed,ast2600-rng
> > +
> > +
> > +  reg:
> > +    description:
> > +      Base address and length of the register set of this block.
> > +      Currently 'reg' must be eight bytes wide and 32-bit aligned.
> > +
> > +    maxItems: 1
> > +
> > +
> > +  period:
> > +    description:
> > +      Wait time in microseconds to be used between reads.
> > +      The RNG on these Aspeed SOCs generates 32bit of random data
> > +      every one microsecond. Choose between 1 and n microseconds.
> > +
> > +    maxItems: 1
> > +
> > +
> > +  mode:
> > +    description:
> > +      One of the eight modes in which the four internal ROs (Ring
> > +      Oscillators)  are combined to generate a stream  of random
> > +      bits. The default mode is seven which is the default method
> > +      of combining RO random bits on these Aspeed SOCs.
> > +
> > +    maxItems: 1
> > +
> > +
> > +  quality:
> > +    description:
> > +      Estimated number of bits of entropy per 1024 bits read from
> > +      the RNG.  Note that the default quality is zero which stops
> > +      this HRNG from automatically filling the kernel's entropy
> > +      pool with data.
> > +
> > +    maxItems: 1
> > +
> > +
> > +required:
> > +  - compatible
> > +  - reg
> > +  - period
> > +  - quality
> > +
> > +
> > +examples:
> > +  - |
> > +    rng: hwrng@1e6e2074 {
> > +         compatible = "aspeed,ast2500-rng";
> > +         reg = <0x1e6e2074 0x8>;
> > +         period = <4>;
> > +         quality = <128>;
> > +         mode = <0x7>;
> > +    };
> > +
> > +
> > +...
> > --
> > 2.17.1