This patch series adds stacked support by enhancing the existing mtd-concat 
driver to be more generic.

As background, a few years ago, Bernhard Frauendienst initiated an effort 
[2] to achieve the same, which was later adapted by Miquel [1] to introduce 
stacked mode support. In this approach, partitions to be concatenated were 
specified using a DT property "part-concat" within the partitions 
definition, allowing two MTD devices to function as a single larger one in 
order to be able to define partitions across chip boundaries. However, the 
bindings were not accepted. As a result, the mtd-concat approach was 
dropped, and alternative DT bindings were introduced [3][4][5], describing 
the two flash devices as one. Corresponding SPI core changes to support 
these bindings were later added [6].

While integrating stacked mode support into SPI-NOR, Tudor provided 
additional feedback, leading to discussions about updating the existing 
DT bindings. To address this, I sent an RFC [7] to initiate discussions on 
adapting the DT bindings as suggested by Miquel in [1]. Following that, 
I am now submitting this patch series that updates the virtual concat DT
bindings and the driver referenced in [1], along with some minor mtdcore 
changes. 
Since I have taken ownership of this effort, I have included Bernhard and 
Miquel under the "Suggested-by" tag.

[1] https://lore.kernel.org/linux-mtd/20191127105522.31445-1-miquel.raynal@bootlin.com/
[2] https://lwn.net/ml/linux-kernel/20180907173515.19990-1-kernel@nospam.obeliks.de/
[3] https://github.com/torvalds/linux/commit/f89504300e94524d5d5846ff8b728592ac72cec4
[4] https://github.com/torvalds/linux/commit/eba5368503b4291db7819512600fa014ea17c5a8
[5] https://github.com/torvalds/linux/commit/e2edd1b64f1c79e8abda365149ed62a2a9a494b4
[6]https://github.com/torvalds/linux/commit/4d8ff6b0991d5e86b17b235fc46ec62e9195cb9b
[7] https://lore.kernel.org/all/20241026075347.580858-1-amit.kumar-mahapatra@amd.com/
---
BRANCH: spi-nor/next

Changes in v14:
 - Updated 1/3 to address review comments.
 - Updated 3/3 to add mtd_virt_concat_destroy() API that would remove a concat
   incase one of the mtd device, which is part of the concat, is removed.
 
Changes in v13:
 - Updated 1/3 to modify the Device Tree binding to support multiple concatenated devices.
 - Added Rob's "Suggested-by" tag in 1/3.
 - Updated 3/3 to define CONFIG_MTD_VIRT_CONCAT as a boolean and depended on CONFIG_MTD_PARTITIONED_MASTER.
 - Updated 3/3 to remove late_initcall and invoke the API directly from mtdcore instead.
 - Updated 3/3 to add support for concatenating more than two MTD partitions.
 - Updated 3/3 to add support for multiple sets of concatenated devices.

Changes in v12:
 - Add stacked mode support throught mtd-concat driver.

Changes in v11:
- Rebased patch series on top of latest for-next branch.
- Added a new patch(1/10) to replace spi->chip_select with
  spi_get_chipselect() call in tps6594-spi.c.
- Added a new patch(2/10) to replace spi->chip_select with
  spi_get_chipseletc() call in cs35l56_hda_spi.c.
- In spi.c initialized unused CS[] to 0xff and spi->cs_index_mask
  to 0x01 in all flows.
- Updated spi_dev_check() to compare the CS of old spi device with
  the new spi device CS.
- Updated cover letter description to add information regarding GPIO CS
  testing and added Stefen's Tested-by tag in 3/10 patch.

Changes in v10:
 - Rebased patch series on top of latest for-next branch and fixed
   merge conflicts.

Changes in v9:
- Updated 1/8 patch description to add an high-level overview of
  parallel(multi-cs) & stacked design.
- Initialized all unused CS to 0xFF.
- Moved CS check from spi_add_device() to __spi_add_device().
- Updated __spi_add_device() to check to make sure that multiple logical CS
  don't map to the same physical CS and same physical CS doesn't map to
  different logical CS.
- Updated 1/8, 5/8 & 7/8 to support arbitrary number of flash devices
  connected in parallel or stacked mode.
- Updated documentation for chip_select.
- Added a new spi-nor structure member nor->num_flash to keep track of the
  number of flashes connected.
- Added a new patch in the series 4/8 to move write_enable call just before
  spi_mem ops call in SPI-NOR.
- Added comments in SPI core & SPI-NOR.
- Rebased the patch series on top of the latest for-next branch.

Changes in v8:
- Updated __spi_add_device() and spi_set_cs() to fix spi driver failure
  with GPIO CS.
- Rebased the patch series on top of latest for-next branch and fixed
  merge conflicts.
- Updated cover letter description to add information regarding GPIO CS
  testing and request Stefan to provide his Tested-by tag for 1/7 patch.
- Updated 1/7 patch description.

Changes in v7:
- Updated spi_dev_check() to avoid failures for spi driver GPIO CS and
  moved the error message from __spi_add_device() to spi_dev_check().
- Resolved code indentation issue in spi_set_cs().
- In spi_set_cs() call spi_delay_exec( ) once if the controller supports
  multi cs with both the CS backed by GPIO.
- Updated __spi_validate()to add checks for both the GPIO CS.
- Replaced cs_index_mask bit mask with SPI_CS_CNT_MAX.
- Updated struct spi_controller to represent multi CS capability of the
  spi controller through a flag bit SPI_CONTROLLER_MULTI_CS instead of
  a boolen structure member "multi_cs_cap".
- Updated 1/7 patch description .

Changes in v6:
- Rebased on top of latest v6.3-rc1 and fixed merge conflicts in
  spi-mpc512x-psc.c, sfdp.c, spansion.c files and removed spi-omap-100k.c.
- Updated spi_dev_check( ) to reject new devices if any one of the
  chipselect is used by another device.

Changes in v5:
- Rebased the patches on top of v6.3-rc1 and fixed the merge conflicts.
- Fixed compilation warnings in spi-sh-msiof.c with shmobile_defconfig

Changes in v4:
- Fixed build error in spi-pl022.c file - reported by Mark.
- Fixed build error in spi-sn-f-ospi.c file.
- Added Reviewed-by: Serge Semin <fancer.lancer@gmail.com> tag.
- Added two more patches to replace spi->chip_select with API calls in
  mpc832x_rdb.c & cs35l41_hda_spi.c files.

Changes in v3:
- Rebased the patches on top of
  https://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi.git for-next
- Added a patch to convert spi_nor_otp_region_len(nor) &
  spi_nor_otp_n_regions(nor) macros into inline functions
- Added Reviewed-by & Acked-by tags

Changes in v2:
- Rebased the patches on top of v6.2-rc1
- Created separate patch to add get & set APIs for spi->chip_select &
  spi->cs_gpiod, and replaced all spi->chip_select and spi->cs_gpiod
  references with the API calls.
- Created separate patch to add get & set APIs for nor->params.
---
Amit Kumar Mahapatra (3):
  dt-bindings: mtd: Describe MTD partitions concatenation
  mtd: Move struct mtd_concat definition to header file
  mtd: Add driver for concatenating devices

 .../bindings/mtd/partitions/partition.yaml    |  20 +
 drivers/mtd/Kconfig                           |  10 +
 drivers/mtd/Makefile                          |   1 +
 drivers/mtd/mtd_virt_concat.c                 | 380 ++++++++++++++++++
 drivers/mtd/mtdconcat.c                       |  12 -
 drivers/mtd/mtdcore.c                         |  17 +
 drivers/mtd/mtdpart.c                         |   6 +
 include/linux/mtd/concat.h                    |  59 +++
 8 files changed, 493 insertions(+), 12 deletions(-)
 create mode 100644 drivers/mtd/mtd_virt_concat.c

-- 
2.34.1

On Mon, 23 Jun 2025 16:24:42 +0530, Amit Kumar Mahapatra wrote:
> This patch series adds stacked support by enhancing the existing mtd-concat
> driver to be more generic.
> 
> As background, a few years ago, Bernhard Frauendienst initiated an effort
> [2] to achieve the same, which was later adapted by Miquel [1] to introduce
> stacked mode support. In this approach, partitions to be concatenated were
> specified using a DT property "part-concat" within the partitions
> definition, allowing two MTD devices to function as a single larger one in
> order to be able to define partitions across chip boundaries. However, the
> bindings were not accepted. As a result, the mtd-concat approach was
> dropped, and alternative DT bindings were introduced [3][4][5], describing
> the two flash devices as one. Corresponding SPI core changes to support
> these bindings were later added [6].
> 
> [...]

Applied to mtd/next, thanks!

[1/3] dt-bindings: mtd: Describe MTD partitions concatenation
      commit: a7c81ac328a6dddf51588ecc7a94f56da34c7875
[2/3] mtd: Move struct mtd_concat definition to header file
      commit: 08be224e3965dc716460ee62ddf1f30421049bed
[3/3] mtd: Add driver for concatenating devices
      commit: fa47dc8295194a03c9182945805b617b01447ae9

Patche(s) should be available on mtd/linux.git and will be
part of the next PR (provided that no robot complains by then).

Kind regards,
Miquèl