linux-kernel - Re: [PATCH] nand_base: optimize checking of erased buffers

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Date:   Wed, 17 May 2017 13:52:09 +0200
From:   Pavel Machek <pavel@....cz>
To:     Mason <slash.tmp@...e.fr>
Cc:     Boris Brezillon <boris.brezillon@...e-electrons.com>,
        Richard Weinberger <richard@....at>,
        David Woodhouse <dwmw2@...radead.org>,
        Brian Norris <computersforpeace@...il.com>,
        Marek Vasut <marek.vasut@...il.com>,
        Cyrille Pitchen <cyrille.pitchen@...ev4u.fr>,
        linux-mtd <linux-mtd@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        mark.marshall@...cronenergy.com, b44839@...escale.com,
        prabhakar@...escale.com
Subject: Re: [PATCH] nand_base: optimize checking of erased buffers


> This is not true for my platform, as the ecc pointer is
> chip->oob_poi + 10
> 
> I suppose it's not a problem if the platform can handle
> unaligned loads, otherwise it spells trouble.
> 
> 
> > +		unsigned long d = *((unsigned long *)bitmap);
> > +		if (d == ~0UL)
> > +			continue;
> > +		weight = hweight_long(d);
> >  		bitflips += BITS_PER_LONG - weight;
> >  		if (unlikely(bitflips > bitflips_threshold))
> >  			return -EBADMSG;
> > 
> 
> The optimization makes sense in itself, but given that it's on an
> error path (?) I'm not sure it will bring any tangible benefits?

Well, if it is critical enough to do magic it does, it is also
important enough not to do expensive bit-counting needlessly... 

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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