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References: <20240227153132.2611499-1-jens.wiklander@linaro.org> <20240227153132.2611499-2-jens.wiklander@linaro.org>
In-Reply-To: <20240227153132.2611499-2-jens.wiklander@linaro.org>
From: Linus Walleij <linus.walleij@linaro.org>
Date: Tue, 5 Mar 2024 13:29:38 +0100
Message-ID: <CACRpkdbhhZ9+Jy_tvWy1RSnyZMjqUhh0ARpS+WRtgWTH3AYCPw@mail.gmail.com>
Subject: Re: [PATCH v3 1/3] rpmb: add Replay Protected Memory Block (RPMB) subsystem
To: Jens Wiklander <jens.wiklander@linaro.org>
Cc: linux-kernel@vger.kernel.org, linux-mmc@vger.kernel.org, 
	op-tee@lists.trustedfirmware.org, 
	Shyam Saini <shyamsaini@linux.microsoft.com>, Ulf Hansson <ulf.hansson@linaro.org>, 
	Jerome Forissier <jerome.forissier@linaro.org>, Sumit Garg <sumit.garg@linaro.org>, 
	Ilias Apalodimas <ilias.apalodimas@linaro.org>, Bart Van Assche <bvanassche@acm.org>, 
	Randy Dunlap <rdunlap@infradead.org>, Ard Biesheuvel <ardb@kernel.org>, Arnd Bergmann <arnd@arndb.de>, 
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Tomas Winkler <tomas.winkler@intel.com>, 
	=?UTF-8?B?QWxleCBCZW5uw6ll?= <alex.bennee@linaro.org>
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Hi Jens,

I realized there is one thing I wonder about:

On Tue, Feb 27, 2024 at 4:31=E2=80=AFPM Jens Wiklander
<jens.wiklander@linaro.org> wrote:

> +struct rpmb_frame {
> +       u8     stuff[196];
> +       u8     key_mac[32];
> +       u8     data[256];
> +       u8     nonce[16];
> +       __be32 write_counter;
> +       __be16 addr;
> +       __be16 block_count;
> +       __be16 result;
> +       __be16 req_resp;
> +} __packed;

I didn't quite get why these things are encoded big-endian?

As on the producer side (the eMMC backend) it seems we are anyway
calling cpu_to_be* to convert them into this format.

If this is a requirement on the consumer side (such as TEE) I think
the consumer should swap the bytes rather than the producer,
but I guess that kind of assumes that we foresee there will be other
consumers in the first place.

Yours,
Linus Walleij