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From: Marek Vasut <marex@denx.de>
To: Boris Brezillon <boris.brezillon@free-electrons.com>
Subject: Re: [PATCH 1/5] mtd: ofpart: grab device tree node directly from
 master device node
Date: Thu, 29 Oct 2015 18:23:47 +0100
Cc: Robert Jarzmik <robert.jarzmik@free.fr>,
 Brian Norris <computersforpeace@gmail.com>, linux-mtd@lists.infradead.org,
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 Ezequiel Garcia <ezequiel.garcia@free-electrons.com>,
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 Kyungmin Park <kyungmin.park@samsung.com>, Han Xu <han.xu@freescale.com>,
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On Thursday, October 29, 2015 at 08:24:48 AM, Boris Brezillon wrote:
> Hi Robert,

Hi!

> On Thu, 29 Oct 2015 07:32:33 +0100
> 
> Robert Jarzmik <robert.jarzmik@free.fr> wrote:
> > Marek Vasut <marex@denx.de> writes:
> > >> Isn't there the case of a single NAND controller with 2 identical
> > >> chips, each a 8 bit NAND chip, and the controller aggregating them to
> > >> offer the OS a single 16-bit NAND chip ?
> 
> Honestly, I don't know how this can possibly work, do you have a real
> example of that use case.
> 
> Here are a few reasons making it impossible:
> 
> 1/ NAND are accessed using specific command sequences, and those
> commands and addresses cycles are sent on through the data bus (AFAIR
> only the lower 8bits of a 16bits bus are used for those
> command/address cycles), so even if you connect the CLE/ALE/CS/RB pins
> on both chips, the one connected on the MSB side of the data bus will
> just receive garbage during the command/address sequences, and your
> program/read operations won't work

Unless you duplicate the command to both MSB and LSB.

> 2/ NAND chips can have bad blocks, so even if you were able to address
> 2 chips (which according to #1 is impossible), you might try to write
> on a bad block on the chip connected on the MSB side of the data bus.

This one is a valid problem. The other valid issue here is where the
command might fail on one chip and pass on the other.

> 3/ There probably are plenty of other reasons why this is not
> possible ;-).

It's possible, implementable, but a really bad idea.

Best regards,
Marek Vasut