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

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Message-ID: <1327670996.26648.43.camel@sauron.fi.intel.com>
Date:	Fri, 27 Jan 2012 15:29:56 +0200
From:	Artem Bityutskiy <dedekind1@...il.com>
To:	Nikolaus Voss <n.voss@...nmann.de>
Cc:	Nicolas Ferre <nicolas.ferre@...el.com>,
	linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mtd: atmel_nand: fix access to 16 bit NAND devices

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?

-- 
Best Regards,
Artem Bityutskiy

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