[PATCH v2] mtd: avoid stack overflow in MTD CFI code

[Arnd Bergmann <arnd@arndb.de>]

When map_word gets too large, we use a lot of kernel stack, and for
MTD_MAP_BANK_WIDTH_32, this means we use more than the recommended
1024 bytes in a number of functions:

drivers/mtd/chips/cfi_cmdset_0020.c: In function 'cfi_staa_write_buffers':
drivers/mtd/chips/cfi_cmdset_0020.c:651:1: warning: the frame size of 1336 bytes is larger than 1024 bytes [-Wframe-larger-than=]
drivers/mtd/chips/cfi_cmdset_0020.c: In function 'cfi_staa_erase_varsize':
drivers/mtd/chips/cfi_cmdset_0020.c:972:1: warning: the frame size of 1208 bytes is larger than 1024 bytes [-Wframe-larger-than=]
drivers/mtd/chips/cfi_cmdset_0001.c: In function 'do_write_buffer':
drivers/mtd/chips/cfi_cmdset_0001.c:1835:1: warning: the frame size of 1240 bytes is larger than 1024 bytes [-Wframe-larger-than=]

This can be avoided if all operations on the map word are done
indirectly and the stack gets reused between the calls. We can
mostly achieve this by selecting MTD_COMPLEX_MAPPINGS whenever
MTD_MAP_BANK_WIDTH_32 is set, but for the case that no other
bank width is enabled, we also need to use a non-constant
map_bankwidth() to convince the compiler to use less stack.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
v2: added 'if HAS_IOMEM' to avoid a Kconfig warning on user-mode-linux.

Originally sent out on Jan 1, sorry for taking my time to follow
up with a new version.

 drivers/mtd/chips/Kconfig |  1 +
 include/linux/mtd/map.h   | 19 +++++++------------
 2 files changed, 8 insertions(+), 12 deletions(-)

diff --git a/drivers/mtd/chips/Kconfig b/drivers/mtd/chips/Kconfig
index 741ec69e0b46..cd162cbc3746 100644
--- a/drivers/mtd/chips/Kconfig
+++ b/drivers/mtd/chips/Kconfig
@@ -117,6 +117,7 @@ config MTD_MAP_BANK_WIDTH_16
 
 config MTD_MAP_BANK_WIDTH_32
 	bool "Support 256-bit buswidth" if MTD_CFI_GEOMETRY
+	select MTD_COMPLEX_MAPPINGS if HAS_IOMEM
 	default n
 	help
 	  If you wish to support CFI devices on a physical bus which is
diff --git a/include/linux/mtd/map.h b/include/linux/mtd/map.h
index 5e0eb7ccabd4..3aa56e3104bb 100644
--- a/include/linux/mtd/map.h
+++ b/include/linux/mtd/map.h
@@ -122,18 +122,13 @@
 #endif
 
 #ifdef CONFIG_MTD_MAP_BANK_WIDTH_32
-# ifdef map_bankwidth
-#  undef map_bankwidth
-#  define map_bankwidth(map) ((map)->bankwidth)
-#  undef map_bankwidth_is_large
-#  define map_bankwidth_is_large(map) (map_bankwidth(map) > BITS_PER_LONG/8)
-#  undef map_words
-#  define map_words(map) map_calc_words(map)
-# else
-#  define map_bankwidth(map) 32
-#  define map_bankwidth_is_large(map) (1)
-#  define map_words(map) map_calc_words(map)
-# endif
+/* always use indirect access for 256-bit to preserve kernel stack */
+# undef map_bankwidth
+# define map_bankwidth(map) ((map)->bankwidth)
+# undef map_bankwidth_is_large
+# define map_bankwidth_is_large(map) (map_bankwidth(map) > BITS_PER_LONG/8)
+# undef map_words
+# define map_words(map) map_calc_words(map)
 #define map_bankwidth_is_32(map) (map_bankwidth(map) == 32)
 #undef MAX_MAP_BANKWIDTH
 #define MAX_MAP_BANKWIDTH 32
-- 
2.7.0