[PATCH 0/3] mtd: point() for cfi_cmdset_0002, on simple maps only

[Orgad Shaneh <orgads@gmail.com>]

jffs2 scans a flash in place through mtd_point() when the chip driver
offers it, and copies every used eraseblock through mtd_read()
otherwise. cfi_cmdset_0002 never had point(), so every jffs2 mount on
an AMD-style NOR copies the whole used area. On an Octeon CN6635 board
with a 100 MB jffs2 partition on an 8-bit M29EW that takes 15-30 s at
every boot.

1/3 tightens the existing gate first. map_is_linear() only checks for
a physical address, so cfi_cmdset_0001 already points at maps whose
own accessors swap bytes (IXP4xx), switch pins (Gemini) or lock the
bus (lantiq). The new map_is_simple() also requires the simple
accessors.

2/3 adds point()/unpoint() to cfi_cmdset_0002 behind the same gate,
modeled on cfi_cmdset_0001, and lists it in the cramfs documentation.

3/3 lets the Octeon flash map use the simple accessors when no eMMC
host shares its boot bus. Without it, 2/3 changes nothing on Octeon.
It builds without 1/3 and 2/3 but only pays off once they are in, so
taking all three through mtd with an ack from Thomas seems simplest.

Tested on that board with 7.2.9: the mount drops to 0.6-0.8 s. The
md5s of the images on the jffs2 partition held across reboots and
across ten 8 MB write/delete rounds. The series applies to v7.3-rc4
and to mtd/next.

Separately, for the MTD maintainers to judge: the write-buffer count
truncation that cfc5ebc9540e ("mtd: cfi_cmdset_0002: cap the
write-buffer chunk at 256 bytes on an x8 device") fixed in
cfi_cmdset_0002 has two siblings. do_write_buffer() in cfi_cmdset_0001
writes CMD(words) and the one in cfi_cmdset_0020 writes
CMD(len / map_bankwidth(map) - 1). On an x8 device CMD() keeps only
the low 8 bits of the count in each device lane, so a write buffer
larger than 256 bytes per device would be programmed with a truncated
count. I have no x8 Intel or ST part to test on, so I have not touched
either; I do not know whether such a part exists.

Orgad Shaneh (3):
  mtd: maps: only point() maps read through the simple accessors
  mtd: cfi_cmdset_0002: implement point() for simple linear maps
  MIPS: Octeon: flash: use the simple map accessors without a shared
    eMMC

 Documentation/filesystems/cramfs.rst  |  11 +-
 arch/mips/cavium-octeon/flash_setup.c |  37 +++++-
 drivers/mtd/chips/cfi_cmdset_0001.c   |   2 +-
 drivers/mtd/chips/cfi_cmdset_0002.c   | 156 +++++++++++++++++++++++++-
 drivers/mtd/maps/map_funcs.c          |  12 ++
 include/linux/mtd/map.h               |   2 +
 6 files changed, 207 insertions(+), 13 deletions(-)

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2.53.0