[PATCH v2 1/4] mtd: nand: mxc_nand: Improve comment about vendor BBM and address verschwurbelung

[Uwe Kleine-König]

To better describe why the BBM is at offset 2000 describe the full
misinterpretation^Wmapping of the NAND memory that the i.MX hardware
implements.

Also adapt the comment to reality: A BBT is created automatically since
commit 2ad441bb7e78 ("mtd: nand-imx: Create BBT automatically when
necessary") which was included in v2020.03.0.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig at pengutronix.de>
---
 drivers/mtd/nand/raw/mxc_nand.c | 32 +++++++++++++++++++++++---------
 1 file changed, 23 insertions(+), 9 deletions(-)

diff --git a/drivers/mtd/nand/raw/mxc_nand.c b/drivers/mtd/nand/raw/mxc_nand.c
index c6533b20fc0f..f7ef2e4374a9 100644
--- a/drivers/mtd/nand/raw/mxc_nand.c
+++ b/drivers/mtd/nand/raw/mxc_nand.c
@@ -1528,20 +1528,32 @@ static const struct nand_controller_ops mxcnd_controller_ops = {
  * 512b data + 16b OOB +
  * 512b data + 16b OOB
  *
+ * So the mapping between original NAND addressing (as intended by the chip
+ * vendor) and interpretation when accessed via the i.MX NAND controller is as
+ * follows:
+ *
+ *       original       |        i.MX
+ * ---------------------+---------------------
+ * data 0x0000 - 0x0200 | data 0x0000 - 0x0200
+ * data 0x0200 - 0x0210 | oob  0x0000 - 0x0010
+ * data 0x0210 - 0x0410 | data 0x0200 - 0x0400
+ * data 0x0410 - 0x0420 | oob  0x0010 - 0x0020
+ * data 0x0420 - 0x0620 | data 0x0400 - 0x0600
+ * data 0x0620 - 0x0630 | oob  0x0020 - 0x0030
+ * data 0x0630 - 0x0800 | data 0x0600 - 0x07d0
+ * oob  0x0000 - 0x0030 | data 0x07d0 - 0x0800
+ * oob  0x0030 - 0x0040 | oob  0x0030 - 0x0040
+ *
  * This means that the factory provided bad block marker ends up
- * in the page data at offset 2000 instead of in the OOB data.
+ * in the page data at offset 2000 = 0x7d0 instead of in the OOB data.
  *
- * To preserve the factory bad block information we take the following
- * strategy:
- *
- * - If the NAND driver detects that no flash BBT is present on 2k NAND
- *   chips it will not create one because it would do so based on the wrong
- *   BBM position
- * - This command is used to create a flash BBT then.
+ * If the NAND driver detects that no flash BBT is present on a 2k NAND
+ * chip it will create one automatically in the assumption that the NAND is
+ * pristine (that is completely erased with only vendor BBMs in the OOB) to
+ * preserve factory bad block information.
  *
  * From this point on we can forget about the BBMs and rely completely
  * on the flash BBT.
- *
  */
 static int checkbad(struct nand_chip *chip, loff_t ofs)
 {
@@ -1562,8 +1574,10 @@ static int checkbad(struct nand_chip *chip, loff_t ofs)
 		return ret;
 
 	if (buf[2000] != 0xff)
+		/* block considered bad */
 		return 1;
 
+	/* block considered good */
 	return 0;
 }
 
-- 
2.43.0