Re: [PATCH 2/2] arm64: dts: imx8mm: Add Crypto CAAM support

From: Schrempf Frieder <frieder.schrempf@kontron.de>
To: Adam Ford <aford173@gmail.com>, Horia Geanta <horia.geanta@nxp.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	"devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
	Aymen Sghaier <aymen.sghaier@nxp.com>,
	"Herbert Xu" <herbert@gondor.apana.org.au>,
	Fabio Estevam <festevam@gmail.com>,
	"Sascha Hauer" <s.hauer@pengutronix.de>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Rob Herring <robh+dt@kernel.org>,
	"linux-crypto@vger.kernel.org" <linux-crypto@vger.kernel.org>,
	"Pengutronix Kernel Team" <kernel@pengutronix.de>,
	Shawn Guo <shawnguo@kernel.org>,
	"David S. Miller" <davem@davemloft.net>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
	dl-linux-imx <linux-imx@nxp.com>
Subject: Re: [PATCH 2/2] arm64: dts: imx8mm: Add Crypto CAAM support
Date: Wed, 11 Dec 2019 14:36:24 +0000	[thread overview]
Message-ID: <fd146818-98c9-7092-5d49-a985db5900c7@kontron.de> (raw)
In-Reply-To: <CAHCN7x+roEAmteNLT9KkLxPvL6AFFHMUW=J_cLcSdE50kODZQQ@mail.gmail.com>

Hi Adam,

On 09.12.19 17:47, Adam Ford wrote:
> On Mon, Dec 9, 2019 at 10:23 AM Horia Geanta <horia.geanta@nxp.com> wrote:
>>
>> On 12/1/2019 12:52 AM, Adam Ford wrote:
>>> The i.MX8M Mini supports the same crypto engine as what is in
>>> the i.MX8MQ, but it is not currently present in the device tree,
>>> because it may be resricted by security features.
>>>
>> What exactly are you referring to?
> 
> I don't know this hardware very well, but on a different platform, we
> needed to make the crypto engines as disabled if they were being
> accessed through secure operations which made it unavailable to Linux
> without using some special barriers. I didn't have the special
> hardware on the other platform that required it that way, so I can't
> really explain it well.  I know on those special cases, because some
> people were accessing these registers through other means, the devices
> had to be marked as 'disabled' so to avoid breaking something.  Since
> I wasn't sure if this was left out of the i.MX8M Mini on purpose, I
> let this disabled just in case this hardware platform was also
> affected in a similar and people wanting to use it could mark it as
> 'okay'

I don't know enough about this to understand the problem you're 
describing. It seems like most SoCs have the CAAM enabled by default in 
the devicetree. On first glance I could only find fsl-lx2160a.dtsi that 
has it disabled.

> 
> adam
> 
>>
>>> This patch places in into the device tree and marks it as disabled,
>>> but anyone not restricting the CAAM with secure mode functions
>>> can mark it as enabled.
>>>
>> Even if - due to export control regulations - CAAM is "trimmed down",
>> it loses only the encryption capabilities (hashing etc. still working).

I don't know much about this, but as Horia said the CAAM might have 
limited capabilities in some cases but would still work.

Therefore I think the CAAM should be enabled by default as it already is 
done for most other SoCs.

Regards,
Frieder

>>
>> Again, please clarify what you mean by "secure mode functions",
>> "security features" etc.
>>
>> Horia
> 
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