linux-kernel - Re: [PATCH] Documentation: dt-bindings: Document 'rng-seed' for /chosen

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Date:   Thu, 2 Apr 2020 11:07:08 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Will Deacon <will@...nel.org>
Cc:     linux-kernel@...r.kernel.org, hsinyi@...omium.org,
        geert+renesas@...der.be, swboyd@...omium.org, robh@...nel.org,
        tytso@....edu
Subject: Re: [PATCH] Documentation: dt-bindings: Document 'rng-seed' for
 /chosen



On 4/2/2020 1:13 AM, Will Deacon wrote:
> Hi Florian,
> 
> On Wed, Apr 01, 2020 at 08:36:40PM -0700, Florian Fainelli wrote:
>> The /chosen node can have a 'rng-seed' property read as a u32 quantity
>> which would contain a random number provided by the boot agent. This is
>> useful in configurations where the kernel does not have access to a
>> random number generator.
>>
>> Signed-off-by: Florian Fainelli <f.fainelli@...il.com>
>> ---
>>  Documentation/devicetree/bindings/chosen.txt | 13 +++++++++++++
>>  1 file changed, 13 insertions(+)
> 
> Thanks for doing this; I realised it was undocumented the other day when I
> tried to look it up myself.
> 
>> diff --git a/Documentation/devicetree/bindings/chosen.txt b/Documentation/devicetree/bindings/chosen.txt
>> index 45e79172a646..126b31eecfeb 100644
>> --- a/Documentation/devicetree/bindings/chosen.txt
>> +++ b/Documentation/devicetree/bindings/chosen.txt
>> @@ -28,6 +28,19 @@ mode) when EFI_RNG_PROTOCOL is supported, it will be overwritten by
>>  the Linux EFI stub (which will populate the property itself, using
>>  EFI_RNG_PROTOCOL).
>>  
>> +rng-seed
>> +--------
>> +
>> +This property is used to initialize the kernel's entropy pool from a
>> +trusted boot agent capable of providing a random number. It is parsed
>> +as a u32 value, e.g.
> 
> Are you sure about this being limited to a u32 value? I thought you could
> pass an arbitrary-length value here.

Humm indeed, we can pass an arbitrary value, I completely conflated the
type of "l" here:

	int l;
	...
        rng_seed = of_get_flat_dt_prop(node, "rng-seed", &l);

with how much we would be reading from the property, which is complete
nonsense. Out of curiosity, what property length do platforms typically
populate?
-- 
Florian