linux-kernel - RE: [PATCH] mtd: atmel_nand: fix access to 16 bit NAND devices

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Message-ID: <EF2E73589CA71846A15D0B2CDF79505D08AD3E5453@wm021.weinmann.com>
Date:	Mon, 30 Jan 2012 08:57:17 +0100
From:	"Voss, Nikolaus" <N.Voss@...nmann.de>
To:	"'dedekind1@...il.com'" <dedekind1@...il.com>
CC:	"'Nicolas Ferre'" <nicolas.ferre@...el.com>,
	"'linux-mtd@...ts.infradead.org'" <linux-mtd@...ts.infradead.org>,
	"'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] mtd: atmel_nand: fix access to 16 bit NAND devices

Artem Bityutskiy wrote on 2012-01-27:
> On Wed, 2012-01-18 at 10:16 +0100, Nikolaus Voss wrote:
>> commit fb5427508abbd635e877fabdf55795488119c2d6 optimizes PIO
>> NAND accesses by using IO memcpy instead of IO read/write
>> repeated functions.
>> 
>> This breaks access to 16 bit NAND devices as memcpy_fromio()/toio()
>> _always_ use byte accesses (see arch/arm/kernel/io.c), so with
>> 16 bit NAND, one byte gets lost per NAND access cycle and NAND
>> address count is wrong.
>> 
>> Using memcpy() instead of the IO memcpy functions fixes this, but
>> depends on correct word alignment of the buffer and length has to
>> be a multiple of four, otherwise it might issue byte accesses and
>> possibly break 16 bit NAND access (cf arch/arm/lib/copy_template.S).
>> 
>> Memcpy variants seem to be the wrong approach here, since the
>> NAND controller doesn't make the NAND appear as truely randomly
>> accessible memory (as opposed to the DRAM controller which does
>> exactly that).
>> 
>> So, my proposal is to use 32 bit IO read/write (and let SMC
>> map it to 8 bit or 16 bit NAND accesses) and account for
>> length % 4 > 0 with two additional IO read/writes.
>> 
>> Signed-off-by: Nikolaus Voss <n.voss@...nmann.de>
> 
> Why not to revert fb5427508abbd635e877fabdf55795488119c2d6 instead, it
> is in my opinion a bit more readable?

No objections. I've tried to save the idea of
fb5427508abbd635e877fabdf55795488119c2d6 but that length % 4 > 0 stuff
uglifies it a little bit...

Niko