linux-kernel - [PATCH] mtd: rawnand: denali: do not pass zero maxchips to nand

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Date:   Tue, 21 Aug 2018 17:23:19 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     linux-mtd@...ts.infradead.org,
        Boris Brezillon <boris.brezillon@...tlin.com>,
        Miquel Raynal <miquel.raynal@...tlin.com>
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        linux-kernel@...r.kernel.org, Marek Vasut <marek.vasut@...il.com>,
        Brian Norris <computersforpeace@...il.com>,
        Richard Weinberger <richard@....at>,
        David Woodhouse <dwmw2@...radead.org>
Subject: [PATCH] mtd: rawnand: denali: do not pass zero maxchips to nand_scan()

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@...ionext.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