From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from metis.ext.pengutronix.de ([2001:6f8:1178:4:290:27ff:fe1d:cc33]) by casper.infradead.org with esmtps (Exim 4.76 #1 (Red Hat Linux)) id 1RbD7u-0006oI-Hr for barebox@lists.infradead.org; Thu, 15 Dec 2011 15:21:43 +0000 Date: Thu, 15 Dec 2011 16:21:33 +0100 From: Sascha Hauer Message-ID: <20111215152133.GT27267@pengutronix.de> References: <1323946179-9883-1-git-send-email-s.hauer@pengutronix.de> <1323946179-9883-8-git-send-email-s.hauer@pengutronix.de> <87y5uevt77.fsf@free.fr> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <87y5uevt77.fsf@free.fr> List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: barebox-bounces@lists.infradead.org Errors-To: barebox-bounces+u.kleine-koenig=pengutronix.de@lists.infradead.org Subject: Re: [PATCH 7/7] nand-bb: implement lseek in readonly mode To: Robert Jarzmik Cc: barebox@lists.infradead.org On Thu, Dec 15, 2011 at 02:51:40PM +0100, Robert Jarzmik wrote: > Sascha Hauer writes: > > > +static off_t nand_bb_lseek(struct cdev *cdev, off_t __offset) > > +{ > > + struct nand_bb *bb = cdev->priv; > > + unsigned long raw_pos = 0; > > + uint32_t offset = __offset; > > + int ret; > > + > > + /* lseek only in readonly mode */ > > + if (bb->flags & O_ACCMODE) > > + return -ENOSYS; > > + > > + while (raw_pos < bb->raw_size) { > > + off_t now = min(offset, bb->info.erasesize); > > + > > + ret = cdev_ioctl(bb->cdev_parent, MEMGETBADBLOCK, (void *)raw_pos); > > + if (ret < 0) > > + return ret; > > + if (!ret) > > + offset -= now; > > + raw_pos += now; > > + if (!offset) { > > + bb->offset = raw_pos; > > + return __offset; > > + } > > + } > Are you sure of this algorithm ? > I tried to check it with: > - erasesize=16 (silly I know, but it's simpler for my mind) > - bb->raw_size = +oo > - offset = 34 > > Let's assume we have eraseblock B0, B1, B2 and B3 of 16 bytes. > B0, B1 and B3 are good, B2 is a bad block. > > +--------+--------+--------+--------+ > | B0 | B1 |xxxB2xxx| B3 | > +--------+--------+--------+--------+ > > If I unroll the while loop: > - loop1: > now=16 > ret=0 (B0 good) > offset = 18 > raw_pos = 16 > - loop2: > now=16 > ret=0 (B1 good) > offset = 2 > raw_pos = 32 > - loop3: > now=2 > ret=1 (B2 bad) > offset = 2 > raw_pos = 34 Yup, that's wrong. We have to add 16 to raw_pos here, not 2. > - loop4: > now=2 > ret=0 (B3 good) > offset = 0 > raw_pos = 36 > bb->offset = 36 > return 34 Funny enough that my code actually works. In loop4 we have ret=1 because we are still on B2 (raw_pos is 34, -> still in block 2). Now the code loops over Block2 in small steps until it comes to block3 and returns with the correct raw offset, so I didn't see this in my tests. > > So we end up with bb->offset = 36, which seems incorrect to me. I would have > understood a value of 50, but 36 ... I don't This one should work like expected: while (raw_pos < bb->raw_size) { off_t now = min(offset, bb->info.erasesize); ret = cdev_ioctl(bb->cdev_parent, MEMGETBADBLOCK, (void *)raw_pos); if (ret < 0) return ret; if (!ret) { offset -= now; raw_pos += now; } else { raw_pos += bb->info.erasesize } if (!offset) { bb->offset = raw_pos; return __offset; } } loop1: now = 16 ret = 0 offset = 18 raw_pos = 16 loop2: now = 16 ret = 0 offset = 2 raw_pos = 32 loop3: now = 2 ret = 1 offset = 2 raw_pos = 48 loop4: now = 2 ret = 0 offset = 0 raw_pos = 50 Thanks for catching this. Sascha -- Pengutronix e.K. | | Industrial Linux Solutions | http://www.pengutronix.de/ | Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 | _______________________________________________ barebox mailing list barebox@lists.infradead.org http://lists.infradead.org/mailman/listinfo/barebox