linux-kernel - Re: [EXT] Re: [PATCH 1/3] dt-bindings: crypto: fsl: add entropy delay property

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Date:   Wed, 29 Jun 2022 15:25:59 -0600
From:   Rob Herring <robh@...nel.org>
To:     Vabhav Sharma <vabhav.sharma@....com>
Cc:     Horia Geanta <horia.geanta@....com>,
        Gaurav Jain <gaurav.jain@....com>,
        Pankaj Gupta <pankaj.gupta@....com>,
        "herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "shawnguo@...nel.org" <shawnguo@...nel.org>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        dl-linux-imx <linux-imx@....com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Silvano Di Ninno <silvano.dininno@....com>,
        Varun Sethi <V.Sethi@....com>
Subject: Re: [EXT] Re: [PATCH 1/3] dt-bindings: crypto: fsl: add entropy
 delay property

On Thu, Jun 16, 2022 at 05:49:30PM +0000, Vabhav Sharma wrote:
> Hello Rob,
> 
> > -----Original Message-----
> > From: Rob Herring <robh@...nel.org>
> > Sent: Monday, June 6, 2022 2:51 AM
> > To: Vabhav Sharma <vabhav.sharma@....com>
> > Cc: Horia Geanta <horia.geanta@....com>; Gaurav Jain
> > <gaurav.jain@....com>; Pankaj Gupta <pankaj.gupta@....com>;
> > herbert@...dor.apana.org.au; davem@...emloft.net;
> > shawnguo@...nel.org; linux-crypto@...r.kernel.org; linux-
> > kernel@...r.kernel.org; dl-linux-imx <linux-imx@....com>;
> > devicetree@...r.kernel.org; linux-arm-kernel@...ts.infradead.org; Silvano Di
> > Ninno <silvano.dininno@....com>; Varun Sethi <V.Sethi@....com>
> > Subject: [EXT] Re: [PATCH 1/3] dt-bindings: crypto: fsl: add entropy delay
> > property
> > 
> > Caution: EXT Email
> > 
> > On Mon, May 30, 2022 at 11:39:22PM +0530, Vabhav Sharma wrote:
> > > Add entropy delay property which defines the length (in system clocks)
> > > of each Entropy sample taken for TRNG configuration.
> > >
> > > Signed-off-by: Vabhav Sharma <vabhav.sharma@....com>
> > > Reviewed-by: Horia Geanta <horia.geanta@....com>
> > > Reviewed-by: Varun Sethi <v.sethi@....com>
> > > ---
> > >  Documentation/devicetree/bindings/crypto/fsl-sec4.txt | 6 ++++++
> > >  1 file changed, 6 insertions(+)
> > >
> > > diff --git a/Documentation/devicetree/bindings/crypto/fsl-sec4.txt
> > > b/Documentation/devicetree/bindings/crypto/fsl-sec4.txt
> > > index 8f359f473ada..1477294eda38 100644
> > > --- a/Documentation/devicetree/bindings/crypto/fsl-sec4.txt
> > > +++ b/Documentation/devicetree/bindings/crypto/fsl-sec4.txt
> > > @@ -62,6 +62,12 @@ PROPERTIES
> > >        Definition: A standard property. Define the 'ERA' of the SEC
> > >            device.
> > >
> > > +   - entropy-delay
> > > +      Usage: optional
> > > +      Value type: <u32>
> > > +      Definition: A property which specifies the length (in system clocks)
> > > +          of each Entropy sample taken.
> > > +
> > 
> > Seems like this could be common, but should be a time value (with unit
> > suffix) rather than clocks. If not common, then needs a vendor prefix.
> > Is this time to read a value or time between values produced? Not really clear
> > from the description.
> CAAM TRNG Configuration includes 16-bit field entropy-delay. This field specifies how long the oscillator is given to freely oscillate and generate a single bit of entropy.
> It is specified as number of system clock cycles and this u32 type field already exist in the caam driver code with default value of 3200. However, on some platform this value can vary and support is added to read the value from device tree in order to override default value, Hope this helps to clarify.

So that is how often a sample can be read? Or what happens if you read a 
sample too quick (in less than this delay time)?

Look at other h/w and drivers see if something common makes sense here.

Rob