linux-kernel - Re: [PATCH] mtd: nand: wait for tWHR after NAND_CMD_STATUS / NAND_CMD

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Message-ID: <20170926102344.6953d072@bbrezillon>
Date:   Tue, 26 Sep 2017 10:23:44 +0200
From:   Boris Brezillon <boris.brezillon@...e-electrons.com>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     Richard Weinberger <richard@....at>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Marek Vasut <marek.vasut@...il.com>,
        linux-mtd@...ts.infradead.org,
        Cyrille Pitchen <cyrille.pitchen@...ev4u.fr>,
        Brian Norris <computersforpeace@...il.com>,
        David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH] mtd: nand: wait for tWHR after NAND_CMD_STATUS /
 NAND_CMD_READID

On Tue, 26 Sep 2017 17:17:49 +0900
Masahiro Yamada <yamada.masahiro@...ionext.com> wrote:

> 2017-09-26 15:43 GMT+09:00 Boris Brezillon <boris.brezillon@...e-electrons.com>:
> > On Tue, 26 Sep 2017 12:39:24 +0900
> > Masahiro Yamada <yamada.masahiro@...ionext.com> wrote:
> >  
> >> Read Status and Read ID require tWHR before reading the first data.
> >> Insert a very short wait to make sure to meet the spec.
> >>
> >> I have not seen any problem report for now, but nand_command() and
> >> nand_command_lP() are generic hooks, so it makes sense to implement
> >> fail-safe code here.
> >>
> >> Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
> >> ---  
> >  
> 
> 
> OK, will do.
> 
> 
> 
> BTW, I see unconditional wait for tWB a few lines below,
> but it gives no performance regression because we will wait
> much longer in nand_wait_ready().

Yep, but this one is here for quite some time. Normally there should be
no problem with your ndelay(200), but I don't want to take the risk
and have someone complain that his NAND controller driver is broken
because of this extra delay ;-).

> 
> 
>      /*
>       * Apply this short delay always to ensure that we do wait tWB in
>       * any case on any machine.
>       */
>      ndelay(100);
> 
>      nand_wait_ready(mtd);
> 
> 
> 
>