Date: Tue, 12 Mar 2024 13:57:39 +0100
From: Ulf Hansson <ulf.hansson@...aro.org>
To: Daniel Golle <daniel@...rotopia.org>
Cc: Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>, Conor Dooley <conor+dt@...nel.org>, 
	Jens Axboe <axboe@...nel.dk>, Dave Chinner <dchinner@...hat.com>, Jan Kara <jack@...e.cz>, 
	Thomas Weißschuh <linux@...ssschuh.net>, 
	Christian Brauner <brauner@...nel.org>, Li Lingfeng <lilingfeng3@...wei.com>, 
	Damien Le Moal <dlemoal@...nel.org>, Min Li <min15.li@...sung.com>, 
	Adrian Hunter <adrian.hunter@...el.com>, Hannes Reinecke <hare@...e.de>, 
	Christian Loehle <CLoehle@...erstone.com>, Avri Altman <avri.altman@....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>, 
	"Ricardo B. Marliere" <ricardo@...liere.net>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-mmc@...r.kernel.org, linux-block@...r.kernel.org, 
	Diping Zhang <diping.zhang@...inet.com>, Jianhui Zhao <zhaojh329@...il.com>, 
	Jieying Zeng <jieying.zeng@...inet.com>, Chad Monroe <chad.monroe@...ran.com>, 
	Adam Fox <adam.fox@...ran.com>, John Crispin <john@...ozen.org>
Subject: Re: [RFC PATCH v2 0/8] nvmem: add block device NVMEM provider

On Tue, 12 Mar 2024 at 13:30, Daniel Golle <daniel@...rotopia.org> wrote:
>
> Hi Ulf,
>
> On Tue, Mar 12, 2024 at 01:22:49PM +0100, Ulf Hansson wrote:
> > On Tue, 5 Mar 2024 at 21:23, Daniel Golle <daniel@...rotopia.org> 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 block devices 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.
> > >
> > > This series is meant to open the discussion on how exactly the device
> > > tree schema for block devices and partitions may look like, and even
> > > if using the block layer to back the NVMEM device is at all the way to
> > > go -- to me it seemed to be a good solution because it will be reuable
> > > e.g. for (normal, software GPT or MBR) partitions of an NVMe SSD.
> > >
> > > This series has previously been submitted on July 19th 2023[1] and most of
> > > the basic idea did not change since.
> > >
> > > However, the recent introduction of bdev_file_open_by_dev() allow to
> > > get rid of most use of block layer internals which supposedly was the
> > > main objection raised by Christoph Hellwig back then.
> > >
> > > Most of the other comments received for in the first RFC have also
> > > been addressed, however, what remains is the use of class_interface
> > > (lacking an alternative way to get notifications about addition or
> > > removal of block devices from the system). As this has been criticized
> > > in the past I'm specifically interested in suggestions on how to solve
> > > this in another way -- ideally without having to implement a whole new
> > > way for in-kernel notifications of appearing or disappearing block
> > > devices...
> > >
> > > And, in a way just like in case of MTD and UBI, I believe acting as an
> > > NVMEM provider *is* a functionality which belongs to the block layer
> > > itself and, other than e.g. filesystems, is inconvenient to implement
> > > elsewhere.
> >
> > I don't object to the above, however to keep things scalable at the
> > block device driver level, such as the MMC subsystem, I think we
> > should avoid having *any* knowledge about the binary format at these
> > kinds of lower levels.
> >
> > Even if most of the NVMEM format is managed elsewhere, the support for
> > NVMEM partitions seems to be dealt with from the MMC subsystem too.
>
> In an earlier iteration of this RFC it was requested to make NVMEM
> support opt-in (instead of opt-out for mtdblock and ubiblock, which
> already got their own NVMEM provider implementation).
> Hence at least a change to opt-in for NVMEM support is required in the
> MMC subsystem, together with making sure that MMC devices have their
> fwnode assigned.

So, the NVMEM support needs to be turned on (opt-in) for each and
every block device driver?

It's not a big deal for me - and I would be happy to apply such a
change. On the other hand, it is just some binary data that is stored
on the flash, why should MMC have to opt-in or opt-out at all? It
should be the upper layers who decide what to store on the flash, not
the MMC subsystem, if you get my point.

>
> > Why can't NVMEM partitions be managed the usual way via the MBR/GPT?
>
> Absolutely, maybe my wording was not clear, but that's exactly what
> I'm suggesting here. There are no added parsers nor any knowledge
> about binary formats in this patchset.

Right, but there are new DT bindings added in the $subject series that
allows us to describe NVMEM partitions for an eMMC. Why isn't that
parsed from the MBR/GPT, etc, rather than encoded in DT?

>
> Or did I misunderstand your comment?

Maybe. I am just trying to understand this, so apologize if you find
my questions silly. :-)

Kind regards
Uffe

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