linux-kernel - Re: [PATCH 0/8] block: implement NVMEM provider

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Date: Fri, 22 Mar 2024 18:02:50 +0000
From: Daniel Golle <daniel@...rotopia.org>
To: Bart Van Assche <bvanassche@....org>
Cc: Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Ulf Hansson <ulf.hansson@...aro.org>, Jens Axboe <axboe@...nel.dk>,
	Dave Chinner <dchinner@...hat.com>, Jan Kara <jack@...e.cz>,
	Thomas Weißschuh <linux@...ssschuh.net>,
	Damien Le Moal <dlemoal@...nel.org>,
	Li Lingfeng <lilingfeng3@...wei.com>,
	Christian Brauner <brauner@...nel.org>,
	Christian Heusel <christian@...sel.eu>,
	Min Li <min15.li@...sung.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Avri Altman <avri.altman@....com>, Hannes Reinecke <hare@...e.de>,
	Christian Loehle <CLoehle@...erstone.com>,
	Bean Huo <beanhuo@...ron.com>, Yeqi Fu <asuk4.q@...il.com>,
	Victor Shih <victor.shih@...esyslogic.com.tw>,
	Christophe JAILLET <christophe.jaillet@...adoo.fr>,
	Dominique Martinet <dominique.martinet@...ark-techno.com>,
	"Ricardo B. Marliere" <ricardo@...liere.net>,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mmc@...r.kernel.org, linux-block@...r.kernel.org
Subject: Re: [PATCH 0/8] block: implement NVMEM provider

On Fri, Mar 22, 2024 at 10:52:17AM -0700, Bart Van Assche wrote:
> On 3/21/24 12:31, Daniel Golle wrote:
> > On embedded devices using an eMMC it is common that one or more (hw/sw)
> > partitions on the eMMC are used to store MAC addresses and Wi-Fi
> > calibration EEPROM data.
> > 
> > Implement an NVMEM provider backed by a block device as typically the
> > NVMEM framework is used to have kernel drivers read and use binary data
> > from EEPROMs, efuses, flash memory (MTD), ...
> > 
> > In order to be able to reference hardware partitions on an eMMC, add code
> > to bind each hardware partition to a specific firmware subnode.
> > 
> > Overall, this enables uniform handling across practially all flash
> > storage types used for this purpose (MTD, UBI, and now also MMC).
> > 
> > As part of this series it was necessary to define a device tree schema
> > for block devices and partitions on them, which (similar to how it now
> > works also for UBI volumes) can be matched by one or more properties.
> 
> Since this patch series adds code that opens partitions and reads
> from partitions, can that part of the functionality be implemented in
> user space? There is already a mechanism for notifying user space about
> block device changes, namely udev.

No. Because it has to happen (e.g. for nfsroot to work) before
userland gets initiated: Without Ethernet MAC address (which if often
stored at some raw offset on a partition or hw-partition of an eMMC),
we don't have a way to use nfsroot (because that requires functional
Ethernet), hence userland won't come up. It's a circular dependency
problem which can only be addressed by making sure that everything
needed for Ethernet to come up is provided by the kernel **before**
rootfs (which can be nfsroot) is mounted.