linux-kernel - Re: [PATCH v2 06/29] mtd: Add support for reading MTD devices via the nvmem API

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Message-ID: <20180821155731.11cae21b@bbrezillon>
Date:   Tue, 21 Aug 2018 15:57:31 +0200
From:   Boris Brezillon <boris.brezillon@...tlin.com>
To:     Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        Rob Herring <robh@...nel.org>
Cc:     Alban <albeu@...e.fr>, Bartosz Golaszewski <brgl@...ev.pl>,
        Jonathan Corbet <corbet@....net>, Sekhar Nori <nsekhar@...com>,
        Kevin Hilman <khilman@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Brian Norris <computersforpeace@...il.com>,
        Marek Vasut <marek.vasut@...il.com>,
        Richard Weinberger <richard@....at>,
        Grygorii Strashko <grygorii.strashko@...com>,
        "David S . Miller" <davem@...emloft.net>,
        Naren <naren.kernel@...il.com>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Lukas Wunner <lukas@...ner.de>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Ivan Khoronzhuk <ivan.khoronzhuk@...aro.org>,
        Sven Van Asbroeck <svendev@...x.com>,
        Paolo Abeni <pabeni@...hat.com>,
        David Lechner <david@...hnology.com>,
        Andrew Lunn <andrew@...n.ch>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-i2c@...r.kernel.org, linux-mtd@...ts.infradead.org,
        linux-omap@...r.kernel.org, netdev@...r.kernel.org,
        Bartosz Golaszewski <bgolaszewski@...libre.com>
Subject: Re: [PATCH v2 06/29] mtd: Add support for reading MTD devices via
 the nvmem API

On Tue, 21 Aug 2018 14:37:37 +0100
Srinivas Kandagatla <srinivas.kandagatla@...aro.org> wrote:

> On 21/08/18 14:34, Srinivas Kandagatla wrote:
> > 
> > 
> > On 21/08/18 12:31, Boris Brezillon wrote:  
> >>>     * struct nvmem_config - NVMEM device configuration
> >>> @@ -58,6 +62,7 @@ struct nvmem_config {
> >>>           bool                    root_only;
> >>>           nvmem_reg_read_t        reg_read;
> >>>           nvmem_reg_write_t       reg_write;
> >>> +       nvmem_match_t           match;
> >>>           int     size;
> >>>           int     word_size;
> >>>           int     stride;
> >>>  
> >> That might work if nvmem cells are defined directly under the mtdnode.  
> > Layout should not matter! which is the purpose of this callback.
> > 
> > The only purpose of this callback is to tell nvmem core that the 
> > node(nvmem cell) belongs to that provider or not, if it is then we 
> > successfully found the provider. Its up to the provider on which layout 
> > it describes nvmem cells. Additionally the provider can add additional 
> > sanity checks in this match function to ensure that cell is correctly 
> > represented.
> > 
> >   
> >> If we go for this approach, I'd recommend replacing this ->match() hook
> >> by ->is_nvmem_cell() and pass it the cell node instead of the nvmem
> >> node, because what we're really after here is knowing which subnode is
> >> an nvmem cell and which subnode is not.  
> > 
> > I agree on passing cell node instead of its parent. Regarding basic 
> > validating if its nvmem cell or not, we can check compatible string in 
> > nvmem core if we decide to use "nvmem-cell" compatible.
> > 
> > Also just in case if you missed this, nvmem would not iterate the  
> Sorry !! i hit send button too quickly I guess.
> 
> What I meant to say here, is that nvmem core would not iterate the 
> provider node in any case.
> 
> Only time it looks at the cell node is when a consumer requests for the 
> cell.

I did miss that, indeed. Thanks for the heads up.

So, the "old partitions being considered as nvmem cells" is not really
a problem, because those parts shouldn't be referenced.
This leaves us with the config->force_compat_check topic, which I'd
like to have to ensure that nvmem cells under MTD nodes actually have
compatible = "nvmem-cell" and prevent people from inadvertently
omitting this prop.

And of course, we need Rob's approval on this new binding :-).