Message-ID: <20160126192645.GA46523@google.com>
Date:	Tue, 26 Jan 2016 11:26:45 -0800
From:	Brian Norris <computersforpeace@...il.com>
To:	Boris Brezillon <boris.brezillon@...e-electrons.com>
Cc:	David Woodhouse <dwmw2@...radead.org>,
	linux-mtd@...ts.infradead.org, Daniel Mack <daniel@...que.org>,
	Haojian Zhuang <haojian.zhuang@...il.com>,
	Robert Jarzmik <robert.jarzmik@...e.fr>,
	Kukjin Kim <kgene@...nel.org>,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	linux-samsung-soc@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Ralf Baechle <ralf@...ux-mips.org>, linux-mips@...ux-mips.org,
	Josh Wu <josh.wu@...el.com>,
	Ezequiel Garcia <ezequiel.garcia@...e-electrons.com>,
	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Chen-Yu Tsai <wens@...e.org>, linux-sunxi@...glegroups.com,
	Stefan Agner <stefan@...er.ch>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org,
	punnaiah choudary kalluri <punnaia@...inx.com>
Subject: Re: [PATCH 00/23] mtd: rework ECC layout definition

Hi Boris,

On Mon, Dec 07, 2015 at 11:25:55PM +0100, Boris Brezillon wrote:
> Hello,
> 
> This patchset aims at getting rid of the nand_ecclayout limitations.
> struct nand_ecclayout is defining fixed eccpos and oobfree arrays which
> can only be increased by modifying the MTD_MAX_ECCPOS_ENTRIES_LARGE and
> MTD_MAX_OOBFREE_ENTRIES_LARGE macros.
> This approach forces us to modify the macro values each time we add a
> new NAND chip with a bigger OOB area, and increasing these arrays also
> penalize all platforms, even those who only support small NAND devices
> (with small OOB area).
> 
> The idea to overcome this limitation, is to define the ECC/OOB layout
> by the mean of two functions: ->eccpos() and ->oobfree(), which will
> basically return the same information has those stored in the
> nand_ecclayout struct.
> 
> Another advantage of this solution is that ECC layouts are usually
> following a repetitive pattern (i.e. leave X bytes free and put Y bytes
> of ECC per ECC chunk), which allows one to implement the ->eccpos()
> and ->oobfree() functions with a simple logic that can be applied
> to any size of OOB.

Thanks for the work! This definitely needed done. I imagined that it
might be best if we just changed the data structure format and have
drivers allocate it dynamically during probe(), but actually, I kinda
like generating it on the fly. The only concern I'd have is if there is
significant penalty to doing this sort of computation on the fly during
(e.g.) AUTO-layout OOB reads/writes. But I guess if there is such a
penalty, nothing would stop us from caching the results in the MTD/NAND
core code.

> Patches 1 to 10 are just cleanups or trivial fixes that can be taken
> independently.

There were some comments for patch 1, and I want to look more closely at
patch 10. But patches 2 to 9 are pushed to l2-mtd.git. Thanks!

> Patch 19 is just an aggregate of several smaller commits (one per
> driver), and has been submitted this way to limit the size of the
> series. If everybody agrees on this approach, I'll resubmit the series
> will those changes separated in different commits (as done here [1]).
> 
> Also note that the last two commits are removing the nand_ecclayout
> definition, thus preventing any new driver to use this structure.
> Of course, this step can be delayed if some of the previous patches
> are not accepted.

I haven't looked in detail at the second half of the series, but I like
the concept. I'll look closer once you fix up things in v2.

Thanks,
Brian

> Best Regards,
> 
> Boris
> 
> [1]https://github.com/bbrezillon/linux-sunxi/commits/nand/ecclayout2

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