mtd: avoid stack overflow in MTD CFI code

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Gitweb:     http://git.infradead.org/?p=mtd-2.6.git;a=commit;h=fddcca5107051adf9e4481d2a79ae0616577fd2c
Commit:     fddcca5107051adf9e4481d2a79ae0616577fd2c
Parent:     20c07a5bf094198ff2382aa5e7c930b3c9807792
Author:     Arnd Bergmann <arnd at arndb.de>
AuthorDate: Mon Feb 29 13:20:28 2016 +0100
Committer:  Brian Norris <computersforpeace at gmail.com>
CommitDate: Sun Apr 3 16:46:24 2016 -0700

    mtd: avoid stack overflow in MTD CFI code
    
    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 at arndb.de>
    [Brian: this patch mostly achieves its goal by forcing
        MTD_COMPLEX_MAPPINGS (and the accompanying indirection) for 256-bit
        mappings; the rest of the change is mostly a wash, though it helps
        reduce stack size slightly. If we really care about supporting
        256-bit mappings though, we should consider rewriting some of this
        code to avoid keeping and assigning so many 256-bit objects on the
        stack.]
    Signed-off-by: Brian Norris <computersforpeace at gmail.com>
---
 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 3b3dabc..bbfa1f1 100644
--- a/drivers/mtd/chips/Kconfig
+++ b/drivers/mtd/chips/Kconfig
@@ -115,6 +115,7 @@ 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 5e0eb7c..3aa56e3 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