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mtd: cfi_cmdset_0001: shrink do_write_buffer() stack frame

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arm32 allmodconfig fails to build with gcc:

  drivers/mtd/chips/cfi_cmdset_0001.c:1883:1: error: the frame size of 1296
  bytes is larger than 1280 bytes [-Werror=frame-larger-than=]

With MTD_MAP_BANK_WIDTH_32 a map_word is 32 bytes, and with KASAN_STACK
every temporary gets its own redzoned slot.  do_write_buffer() builds a
map_word for each of the twelve commands it issues, and those temporaries
dominate its frame.

Issue the commands through a small noinline helper that takes the
command as a plain value and builds the map_word in its own frame.  The
sequence of reads and writes to the chip is unchanged.

The helper must not be inlined, and reusing one map_word local is not
enough: clang cannot assume the callee does not observe the local, so it
still returns each CMD() into a per-call-site temporary, and the frame
grows (1760 -> 1824 bytes in cfi_intelext_writev(), into which clang
inlines do_write_buffer()).

Frame sizes with MTD_MAP_BANK_WIDTH_32 and KASAN_STACK:

  gcc 15.2   do_write_buffer()      1296 -> 520
  clang 21   cfi_intelext_writev()  1760 -> 896

The helper itself takes 168 (gcc) / 160 (clang) bytes.

Reported-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Closes: https://lore.kernel.org/all/20260408211407.2295175-1-andriy.shevchenko@linux.intel.com/
Suggested-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/all/87ik9cfm6g.fsf@bootlin.com/
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
This commit is contained in:
Karl Mehltretter 2026-08-24 03:47:03 +02:00 • committed by Miquel Raynal
commit 39b975ff20
1 changed files with 26 additions and 16 deletions
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