linux-kernel - Re: [RFC PATCH 6/6] block: implement NVMEM provider

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Date:   Fri, 21 Jul 2023 13:11:40 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Daniel Golle <daniel@...rotopia.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Richard Weinberger <richard@....at>,
        Vignesh Raghavendra <vigneshr@...com>,
        Dave Chinner <dchinner@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Thomas Weißschuh <linux@...ssschuh.net>,
        Jan Kara <jack@...e.cz>, Damien Le Moal <dlemoal@...nel.org>,
        Ming Lei <ming.lei@...hat.com>, Min Li <min15.li@...sung.com>,
        Christian Loehle <CLoehle@...erstone.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Hannes Reinecke <hare@...e.de>,
        Jack Wang <jinpu.wang@...os.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Yeqi Fu <asuk4.q@...il.com>, Avri Altman <avri.altman@....com>,
        Hans de Goede <hdegoede@...hat.com>,
        Ye Bin <yebin10@...wei.com>,
        Rafał Miłecki <rafal@...ecki.pl>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mmc@...r.kernel.org, linux-mtd@...ts.infradead.org
Subject: Re: [RFC PATCH 6/6] block: implement NVMEM provider

On Fri, Jul 21, 2023 at 11:40:51AM +0100, Daniel Golle wrote:
> On Thu, Jul 20, 2023 at 11:31:06PM -0700, Christoph Hellwig wrote:
> > On Thu, Jul 20, 2023 at 05:02:32PM +0100, Daniel Golle wrote:
> > > On Thu, Jul 20, 2023 at 12:04:43AM -0700, Christoph Hellwig wrote:
> > > > The layering here is exactly the wrong way around.  This block device
> > > > as nvmem provide has not business sitting in the block layer and being
> > > > keyed ff the gendisk registration.  Instead you should create a new
> > > > nvmem backed that opens the block device as needed if it fits your
> > > > OF description without any changes to the core block layer.
> > > > 
> > > 
> > > Ok. I will use a class_interface instead.
> > 
> > I'm not sure a class_interface makes much sense here.  Why does the
> > block layer even need to know about you using a device a nvmem provider?
> 
> It doesn't. But it has to notify the nvmem providing driver about the
> addition of new block devices. This is what I'm using class_interface
> for, simply to hook into .add_dev of the block_class.

Why is this single type of block device special to require this, yet all
others do not?  Encoding this into the block layer feels like a huge
layering violation to me, why not do it how all other block drivers do
it instead?

> > As far as I can tell your provider should layer entirely above the
> > block layer and not have to be integrated with it.
> 
> My approach using class_interface doesn't require any changes to be
> made to existing block code. However, it does use block_class. If
> you see any other good option to implement matching off and usage of
> block devices by in-kernel users, please let me know.

Do not use block_class, again, that should only be for the block core to
touch.  Individual block drivers should never be poking around in it.

thanks,

greg k-h