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

From: Boris Brezillon <boris.brezillon@free-electrons.com>
To: Brian Norris <computersforpeace@gmail.com>
Cc: linux-mtd@lists.infradead.org, linux-kernel@vger.kernel.org,
	Ezequiel Garcia <ezequiel.garcia@free-electrons.com>,
	Marek Vasut <marex@denx.de>, Scott Wood <scottwood@freescale.com>,
	Josh Wu <josh.wu@atmel.com>,
	Robert Jarzmik <robert.jarzmik@free.fr>,
	Kyungmin Park <kyungmin.park@samsung.com>,
	Han Xu <han.xu@freescale.com>,
	Huang Shijie <shijie.huang@arm.com>
Subject: Re: [PATCH 1/5] mtd: ofpart: grab device tree node directly from master device node
Date: Wed, 28 Oct 2015 09:02:23 +0100	[thread overview]
Message-ID: <20151028090223.4564a966@bbrezillon> (raw)
In-Reply-To: <20151028010102.GB13239@google.com>

Hi Brian,

On Tue, 27 Oct 2015 18:01:02 -0700
Brian Norris <computersforpeace@gmail.com> wrote:

> On Tue, Oct 27, 2015 at 10:54:46AM -0700, Brian Norris wrote:
> > On Tue, Oct 27, 2015 at 08:42:00AM +0100, Boris Brezillon wrote:
> > > On Mon, 26 Oct 2015 19:31:06 -0700
> > > 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()?
> 
> I looked at this quickly for NAND, and it's hard to do right now because
> of the below quote. The SPI NOR layering is better though, so that
> works. Mind if I defer the dropping the flash_node in NAND but do the
> SPI NOR one?

No, I don't mind (we'll solve that later).

> 
> > 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?
> 
> The layering (or lack thereof) make it hard to extract a struct mtd_info
> from a struct nand_chip.

That's true, and that's why we'd better rework a few things (embed
or point to an allocated mtd struct in nand_chip) before trying to
automatically assign mtd->priv to a nand_chip pointer.

Best Regards,

Boris

-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com