Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <318c0814-7f0b-9798-6998-5039094b010d@gmail.com>
Date:   Thu, 5 May 2022 07:46:34 +0200
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:96.0) Gecko/20100101
 Thunderbird/96.0
Subject: Re: [PATCH V2] nvmem: add driver handling U-Boot environment
 variables
To:     Ahmad Fatoum <a.fatoum@pengutronix.de>,
        Srinivas Kandagatla <srinivas.kandagatla@linaro.org>,
        Miquel Raynal <miquel.raynal@bootlin.com>,
        Richard Weinberger <richard@nod.at>,
        Vignesh Raghavendra <vigneshr@ti.com>
Cc:     Tom Rini <trini@konsulko.com>, linux-mtd@lists.infradead.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        u-boot@lists.denx.de, devicetree@vger.kernel.org,
        =?UTF-8?B?UmFmYcWCIE1pxYJlY2tp?= <rafal@milecki.pl>,
        Pengutronix Kernel Team <kernel@pengutronix.de>
References: <20220503165658.13932-1-zajec5@gmail.com>
 <79c7891a-9a68-a111-094d-be9804071a9e@pengutronix.de>
From:   =?UTF-8?B?UmFmYcWCIE1pxYJlY2tp?= <zajec5@gmail.com>
In-Reply-To: <79c7891a-9a68-a111-094d-be9804071a9e@pengutronix.de>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Precedence: bulk

On 4.05.2022 11:23, Ahmad Fatoum wrote:
> Hello Rafał,
> 
> On 03.05.22 18:56, Rafał Miłecki wrote:
>> From: Rafał Miłecki <rafal@milecki.pl>
>>
>> U-Boot stores its setup as environment variables. It's a list of
>> key-value pairs stored on flash device with a custom header.
>>
>> This commit adds an NVMEM driver that:
>> 1. Provides NVMEM access to environment vars binary data
>> 2. Extracts variables as NVMEM cells
>>
>> It can be used for:
>> 1. Accessing env variables from user-space
> 
> Is this already possible? The only interface I know of is the /nvmem
> file in sysfs, but that one is not per cell, but per device.

Maybe that wasn't precise enough, I should probably write:
1. Parsing binary data from user-space

In future I'd like to extend U-Boot's "printenv" tool to support reading
env variables blob using Linux's sysfs as documented in the
Documentation/ABI/stable/sysfs-bus-nvmem


>> +	label = of_get_property(np->parent, "label", NULL);
>> +	if (!label)
>> +		label = np->parent->name;
>> +
>> +	priv->mtd = get_mtd_device_nm(label);
>> +	if (IS_ERR(priv->mtd)) {
>> +		dev_err(dev, "Failed to find \"%s\" MTD device: %ld\n", label, PTR_ERR(priv->mtd));
>> +		return PTR_ERR(priv->mtd);
>> +	}
> 
> I am trying to make sense of this using the binding, but I can't.
> Do you have an example device tree fragment?

This comes from unreleased yet board I'm working on.

It stores U-Boot env variables in the middle of U-Boot binary.

partitions {
	compatible = "fixed-partitions";
	#address-cells = <1>;
	#size-cells = <1>;

	partition@0 {
		label = "loader";
		reg = <0x0 0x100000>;

		partition@40000 {
			compatible = "u-boot,env";
			label = "u-boot-env";
			reg = <0x40000 0x4000>;
		};
	};

	partition@100000 {
		label = "image";
		reg = <0x100000 0x1fe00000>;
	};
};