linux-kernel - questions about [PATCH]mtd: nand: fix SCAN2NDPAGE check for BBM

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Date:	Tue, 13 Oct 2015 11:33:54 +0000
From:	Yanjiantao <yanjiantao@...ilicon.com>
To:	"computersforpeace@...il.com" <computersforpeace@...il.com>
CC:	"linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Caizhiyong <caizhiyong@...ilicon.com>,
	"Wanli (welly)" <welly.wan@...ilicon.com>,
	Quyaxin <quyaxin@...ilicon.com>
Subject: questions about  [PATCH]mtd: nand: fix SCAN2NDPAGE check for BBM

Dear Brian,

i read the patch:  " mtd: nand: fix SCAN2NDPAGE check for BBM "
And i am confused about the markbad flows.

		xxxxx;
        if (chip->bbt_options & NAND_BBT_SCANLASTPAGE)
                ofs += mtd->erasesize - mtd->writesize;

		xxxxx;

        do { 
                if (chip->options & NAND_BUSWIDTH_16) {
                        chip->cmdfunc(mtd, NAND_CMD_READOOB,
                                        chip->badblockpos & 0xFE, page);
                        bad = cpu_to_le16(chip->read_word(mtd));
                        if (chip->badblockpos & 0x1) 
                                bad >>= 8;
                        else 
                                bad &= 0xFF;
                } else {
                        chip->cmdfunc(mtd, NAND_CMD_READOOB, chip->badblockpos,
                                        page);
                        bad = chip->read_byte(mtd);
                }    

                if (likely(chip->badblockbits == 8))
                        res = bad != 0xFF;
                else 
                        res = hweight8(bad) < chip->badblockbits;
                ofs += mtd->writesize;
                page = (int)(ofs >> chip->page_shift) & chip->pagemask;
                i++; 
        } while (!res && i < 2 && (chip->bbt_options & NAND_BBT_SCAN2NDPAGE)); 
mark bad flows is the same.

If I understand the patch right, then I think that:
1. when NAND_BBT_SCANLASTPAGE is set, markbad and checkbad on the last page only.
2. when NAND_BBT_SCAN2NDPAGE is set, markbad and checkbad on the 1st and 2nd page.

I checked some datasheets of different manufacturers(such as SAMSUNG/HYNIX/MICRON/TOSHIBA), they claim that bad block marker is on 1st/2nd, or 1st/last, or 1st/2nd/last(for spansion).
the checkbad method may check fail in some cases:
for example,:
1. for spansion nand(bad block marker is on 1st or 2nd or last page), set NAND_BBT_SCAN2NDPAGE while badblock marker is on last page or set NAND_BBT_SCANLASTPAGE while badblock marker is on 1st/2nd page may cause badblock chech failed.
2. for HYNIX (H27UBG8T2CTR) nand (bad block marker is on 1st or last page), check may failed whether NAND_BBT_SCANLASTPAGE is set or not.

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