linux-kernel - Re: fsl_ifc_nand: are blank pages protected by ECC?

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Message-ID: <20170420114057.GA4705@amd>
Date:   Thu, 20 Apr 2017 13:40:57 +0200
From:   Pavel Machek <pavel@....cz>
To:     Boris Brezillon <boris.brezillon@...e-electrons.com>
Cc:     richard@....at, dwmw2@...radead.org, computersforpeace@...il.com,
        marek.vasut@...il.com, cyrille.pitchen@...el.com,
        linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org,
        mark.marshall@...cronenergy.com, b44839@...escale.com,
        prabhakar@...escale.com
Subject: Re: fsl_ifc_nand: are blank pages protected by ECC?

Hi!

> > Would it make sense to only do hweight if *bitmap != ~0ULL ? Would it
> > make sense to only check for bitflips > bitflips_threshold each 128
> > bytes or something like that?
> 
> I didn't go as far as you did and simply assumed hweight32/64() were
> already optimized. Feel free to propose extra improvements.

I'd propose this one (only compile tested, sorry, not sure how to test
this one). If we see ~0UL, there's no need for hweight, and no need to
check number of bitflips. So this should be net win.

Signed-off-by: Pavel Machek <pavel@...x.de>

diff --git a/drivers/mtd/nand/nand_base.c b/drivers/mtd/nand/nand_base.c
index b0524f8..96c27ec 100644
--- a/drivers/mtd/nand/nand_base.c
+++ b/drivers/mtd/nand/nand_base.c
@@ -1357,7 +1357,10 @@ static int nand_check_erased_buf(void *buf, int len, int bitflips_threshold)
 
 	for (; len >= sizeof(long);
 	     len -= sizeof(long), bitmap += sizeof(long)) {
-		weight = hweight_long(*((unsigned long *)bitmap));
+		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;

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

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