linux-kernel - Re: [PATCH v6 00/18] mtd: nand: denali: Denali NAND IP patch bomb

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Message-ID: <20170613103043.622a2c98@bbrezillon>
Date:   Tue, 13 Jun 2017 10:30:43 +0200
From:   Boris Brezillon <boris.brezillon@...e-electrons.com>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     Cyrille Pitchen <cyrille.pitchen@...ev4u.fr>,
        Richard Weinberger <richard@....at>,
        Marek Vasut <marek.vasut@...il.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Chuanxiao Dong <chuanxiao.dong@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Dinh Nguyen <dinguyen@...nel.org>,
        linux-mtd@...ts.infradead.org,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>,
        Jassi Brar <jaswinder.singh@...aro.org>,
        Brian Norris <computersforpeace@...il.com>,
        Enrico Jorns <ejo@...gutronix.de>
Subject: Re: [PATCH v6 00/18] mtd: nand: denali: Denali NAND IP patch bomb

On Tue, 13 Jun 2017 17:17:41 +0900
Masahiro Yamada <yamada.masahiro@...ionext.com> wrote:

> Hi Boris,
> 
> 
> 2017-06-13 16:02 GMT+09:00 Boris Brezillon <boris.brezillon@...e-electrons.com>:
> 
> >
> > BTW, I also implemented ->read/write_buf_word() since the core may one
> > day call these functions, and the default implementations used by the
> > core when these hooks are NULL are not appropriate in your case.
> >  
> 
> BTW, why doesn't the default hook in the core do like this?
> 
> 
> static uint8_t nand_read_byte(struct mtd_info *mtd)
> {
>         struct nand_chip *chip = mtd_to_nand(mtd);
>         uint8_t byte;
> 
>         chip->read_buf(chip, &byte, 1);
>         return byte;
> }
> 
> 
> ->read_byte() is a special case of ->read_buf() with length==1,  
> so this should work.

Not sure it works for all implementation. ->read_byte() is expected
to return the lower 8 bits when interacting with a 16-bits bus. If we do
what you suggest, and ->read_buf() appears to be caching data in an
intermediate buffer if the amount of data is not aligned on 2 bytes,
you might retrieve data you don't care about when ->read_byte() is
called several times.