Re: nvmem: Defining cells on mtd created by mtdparts

From: Pratyush Yadav <p.yadav@ti.com>
To: Sven Eckelmann <sven@narfation.org>
Cc: <openwrt-devel@lists.openwrt.org>,
	Ansuel Smith <ansuelsmth@gmail.com>,
	Michael Walle <michael@walle.cc>,
	Adrian Schmutzler <dev@schmutzler.it>,
	Srinivas Kandagatla <srinivas.kandagatla@linaro.org>,
	<linux-mtd@lists.infradead.org>,
	Bartosz Golaszewski <bgolaszewski@baylibre.com>,
	<linux-kernel@vger.kernel.org>
Subject: Re: nvmem: Defining cells on mtd created by mtdparts
Date: Tue, 12 Oct 2021 23:54:46 +0530	[thread overview]
Message-ID: <20211012182444.qrn3lzp7vukklwlx@ti.com> (raw)
In-Reply-To: <14722734.oMan5NXi5u@sven-desktop>

Hi Sven,

On 11/10/21 09:06AM, Sven Eckelmann wrote:
> On Sunday, 10 October 2021 14:53:13 CEST Sven Eckelmann wrote:
> [...]
> > Since there are most likely more devices out there which use mtdparts, I would 
> > guess that there might already be a strategy out there which can be used to 
> > define the nvmem-provider for mtdparts defined partitions. At least I saw that 
> > Bartosz Golaszewski added all the mtd devices automatically as nvmem provider 
> > in c4dfa25ab307 ("mtd: add support for reading MTD devices via the nvmem 
> > API"). So there might also be something for nvmem-cells to find the correct 
> > mtd instead of relying on the fixed-partitions registration + of_node (which 
> > doesn't exist because it comes from mtdparts and not devicetree).

I have been wanting to fix this problem for a while but just never got 
around to it. I was thinking about either extending the mtdparts syntax 
to maybe add nvmem cell information in there or adding a separate 
cmdline argument that complements mtdparts with nvmem cell info. Dunno 
if either of these would work well though...

> 
> Ansuel Smith just proposed in OpenWrt [1] a workaround. It basically adds a 
> minimal fixed-partitions parser to the mtd cmdlinepart parser (responsible for 
> the mtdparts=) that tries to find the matching (size + offset) fixed-partition 
> from the devicetree. The code in mtd_device_parse_register
> (add_mtd_partitions -> add_mtd_device -> mtd_nvmem_add) will then 
> automatically take care of the rest.

I don't quite see how this helps. You say that some devices don't have a 
device tree at all so how would you even match the fixed partition? And 
this of course doesn't solve the problem where you might want nvmem 
cells with a partition layout that is different from the one in device 
tree.

I unfortunately don't really have any answers for this at this point, 
but maybe I can figure something out in the future...

> 
> Kind regards,
> 	Sven
> 
> [1] https://github.com/openwrt/openwrt/pull/4664#issuecomment-939567963

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-- 
Regards,
Pratyush Yadav
Texas Instruments Inc.