linux-kernel - Re: [PATCH 1/5] mtd: ofpart: grab device tree node directly from master device node

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Date:	Tue, 27 Oct 2015 10:54:46 -0700
From:	Brian Norris <computersforpeace@...il.com>
To:	Boris Brezillon <boris.brezillon@...e-electrons.com>
Cc:	linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Ezequiel Garcia <ezequiel.garcia@...e-electrons.com>,
	Marek Vasut <marex@...x.de>,
	Scott Wood <scottwood@...escale.com>,
	Josh Wu <josh.wu@...el.com>,
	Robert Jarzmik <robert.jarzmik@...e.fr>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Han Xu <han.xu@...escale.com>,
	Huang Shijie <shijie.huang@....com>
Subject: Re: [PATCH 1/5] mtd: ofpart: grab device tree node directly from
 master device node

Hi Boris,

On Tue, Oct 27, 2015 at 08:42:00AM +0100, Boris Brezillon wrote:
> On Mon, 26 Oct 2015 19:31:06 -0700
> Brian Norris <computersforpeace@...il.com> wrote:
> 
> > It seems more logical to use a device node directly associated with the
> > MTD master device (i.e., mtd->dev.of_node field) rather than requiring
> > auxiliary partition parser information to be passed in by the driver in
> > a separate struct.
> > 
> > This patch supports the mtd->dev.of_node field, deprecates the parser
> > data 'of_node' field, and begins using the new convention for nand_base.
> > Other NAND driver conversions may now follow.
> > 
> > Additional side benefit to assigning mtd->dev.of_node rather than using
> > parser data: the driver core will automatically create a device -> node
> > symlink for us.
> 
> I like the idea, but how about pushing the solution even further and
> killing the ->flash_node field which AFAICT is rendered useless by
> your patch?

I suppose we could do that. I do think there's something to be said for
layering, though. Historically, we haven't done a very good job of
layering in MTD, so low-level drivers often have to poke around in the
MTD structures, even if they really should only have to know a few
things about their helper subsystem/library, like NAND or SPI NOR. So
with that in mind, I think the ->flash_node serves some purpose --
drivers can just initialize struct nand_chip/spi_nor and be assured that
the NAND/SPI-NOR subsystems will take care of things.

Now, I don't think there's much reason to suspect that we'd have a more
complex mapping than 1:1 between struct mtd_info and struct nand_chip or
struct spi_nor, so maybe we don't actually need duplicate storage
(mtd.dev.of_node and {spi_nor,nand_chip}.flash_node), and the layering
is just have these APIs:

	nand_set_flash_node()
	spi_nor_set_flash_node()

which just call mtd_set_of_node()?

Speaking of layering: why do we have NAND drivers initializing mtd->priv
for us, yet nand_base just assumes that it points to a struct nand_chip?
And why isn't struct mtd_info just embedded in struct nand_chip? Are
there ever cases we want more than one (master) MTD per nand_chip? Or
vice versa?

Thanks for the review,
Brian
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