[PATCH] mtd: rawnand: denali: do not pass zero maxchips to nand_scan()

Masahiro Yamada
2018-08-21T08:24:57Z

Commit 49aa76b16676 ("mtd: rawnand: do not execute nand_scan_ident()
if maxchips is zero") gave a new meaning for calling nand_scan_ident()
with maxchips=0.

It is a special usage for some drivers such as docg4, but in fact
the Denali driver may pass maxchips=0 to nand_scan() when the driver
is enabled but no NAND chip is found on the board for some reasons.

If nand_scan_with_ids() is called with maxchips=0, nand_scan_ident()
is skipped, i.e. nand_set_defaults() is skipped. Therefore, the
driver must have set chip->controller beforehand. Otherwise,
nand_attach() causes NULL pointer dereference.

In fact, the Denali controller knows the number of connected chips
before calling nand_scan_ident(); if DEVICE_RESET fails, there is no
chip in that chip select. Then, denali_reset_banks() sets the maxchips
to the number of detected chips. If no chip is found, it is zero.

The reason of this trick was, as commit f486287d2372 ("mtd: nand:
denali: fix bank reset function to detect the number of chips")
explained, nand_scan_ident() issued Set Features (0xEF) command
to all CS lines, some of which may not be connected with a chip.
Then, the driver would wait until R/B# response, which never happens.

This problem was solved by commit 107b7d6a7ad4 ("mtd: rawnand: avoid
setting again the timings to mode 0 after a reset"). In the current
code, nand_setup_data_interface() is called from nand_scan_tail(),
which is after the chip detection is done.

Remove the code that is causing NULL pointer dereference. Now, the
maxchips passed to nand_scan() is the maximum number of chip selects
supported by the IP (typically 4 or 8). Leave all the chip detection
process to nand_scan_ident().

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
---

drivers/mtd/nand/raw/denali.c | 1 -
1 file changed, 1 deletion(-)

diff --git a/drivers/mtd/nand/raw/denali.c b/drivers/mtd/nand/raw/denali.c
index ca18612..3e4b8e1 100644
--- a/drivers/mtd/nand/raw/denali.c
+++ b/drivers/mtd/nand/raw/denali.c
@@ -1086,7 +1086,6 @@ static void denali_reset_banks(struct denali_nand_info *denali)
}

dev_dbg(denali->dev, "%d chips connected\n", i);
- denali->max_banks = i;
}

static void denali_hw_init(struct denali_nand_info *denali)
--
2.7.4