linux-kernel - Re: [PATCH v2 1/4] nvmem: nintendo-otp: Add new driver for the Wii and Wii U OTP

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Date:   Sat, 26 Jun 2021 23:24:07 +0000
From:   Jonathan Neuschäfer <j.ne@...teo.net>
To:     Emmanuel Gil Peyrot <linkmauve@...kmauve.fr>
Cc:     Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        linuxppc-dev@...ts.ozlabs.org, devicetree@...r.kernel.org,
        Ash Logan <ash@...quark.com>,
        Jonathan Neuschäfer <j.ne@...teo.net>,
        Rob Herring <robh+dt@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/4] nvmem: nintendo-otp: Add new driver for the Wii
 and Wii U OTP

Hi,


On Wed, May 19, 2021 at 11:50:41AM +0200, Emmanuel Gil Peyrot wrote:
> This OTP is read-only and contains various keys used by the console to
> decrypt, encrypt or verify various pieces of storage.
> 
> Its size depends on the console, it is 128 bytes on the Wii and
> 1024 bytes on the Wii U (split into eight 128 bytes banks).
> 
> It can be used directly by writing into one register and reading from
> the other one, without any additional synchronisation.
> 
> Signed-off-by: Emmanuel Gil Peyrot <linkmauve@...kmauve.fr>
> ---

A link to the (third-party) documentation would be nice, either in the
commit message or in the code itself.

(https://wiiubrew.org/wiki/Hardware/OTP i guess)

[...]
> +static int nintendo_otp_reg_read(void *context,
> +				 unsigned int reg, void *_val, size_t bytes)
> +{
> +	struct nintendo_otp_priv *priv = context;
> +	u32 *val = _val;
> +	int words = bytes >> 2;
> +	u32 bank, addr;
> +
> +	while (words--) {
> +		bank = (reg << 1) & ~0xff;

This is a bit non-obvious, IMHO. As far as I understand it, the expanded
formula is:

	bank = (reg / 128) << 8;

I.e. first divide by bank size, then shift the parameter into the right
place.

> +		addr = (reg >> 2) & 0x1f;

Here, I think it's about the word size (4 bytes); I think / 4 would be
clearer.

I *think* (but haven't checked) that gcc should generate efficent shifts
for the divisions above, so using the division operator shouldn't be
problem.

> +		iowrite32be(OTP_READ | bank | addr, priv->regs + HW_OTPCMD);
> +		*val++ = ioread32be(priv->regs + HW_OTPDATA);
> +		reg += 4;
> +	}
> +
> +	return 0;
> +}
> +
[...]
> +	if (of_id->data) {
> +		const struct nintendo_otp_devtype_data *data = of_id->data;
> +		config.name = data->name;
> +		config.size = data->num_banks * 128;

Given that 128 appears a few times, perhaps a #define would be good.

> +	}
> +
> +	config.dev = dev;
> +	config.priv = priv;
> +
> +	nvmem = devm_nvmem_register(dev, &config);
> +
> +	return PTR_ERR_OR_ZERO(nvmem);
> +}
> +
> +static struct platform_driver nintendo_otp_driver = {
> +	.probe = nintendo_otp_probe,
> +	.driver = {
> +		.name = "nintendo-otp",
> +		.of_match_table = nintendo_otp_of_table,
> +	},
> +};
> +module_platform_driver(nintendo_otp_driver);
> +MODULE_AUTHOR("Emmanuel Gil Peyrot <linkmauve@...kmauve.fr>");
> +MODULE_DESCRIPTION("Nintendo Wii and Wii U OTP driver");
> +MODULE_LICENSE("GPL v2");
> -- 
> 2.31.1
> 

Tested-by: Jonathan Neuschäfer <j.ne@...teo.net>  # on Wii



Thanks,
Jonathan Neuschäfer

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