[RFC PATCH 0/2] Add support for stacked and parallel memories

[Mahapatra, Amit Kumar]

Hello Tudor, Michael, Mark, Rob, Pratyush & Krzysztof

Please share your thoughts on the proposed binding changes and overall 
architecture, so we can discuss and finalize the same.

Regards,
Amit

> Subject: [RFC PATCH 0/2] Add support for stacked and parallel memories
> 
> Hello Everyone,
> 
> Following an email discussion with Miquel regarding the binding changes and overall
> architecture for implementing support for stacked and parallel memories, I’m sharing
> this RFC to initiate a discussion on the proposed updates to current bindings and to
> finalize the implementation architecture.
> 
> Before diving into the main topic, here is some background on stacked and parallel
> memories.
> 
> The AMD QSPI controller supports two advanced connection modes(Stacked and
> Parallel) which allow the controller to treat two different flashes as one storage.
> 
> Stacked:
> Flashes share the same SPI bus, but different CS line, controller driver asserts the
> CS of the flash to which it needs to communicate. Stacked mode is a software
> abstraction rather than a controller feature or capability.
> At any given time, the controller communicates with one of the two connected flash
> devices, as determined by the requested address and data length. If an operation
> starts on one flash and ends on the other, the core needs to split it into two separate
> operations and adjust the data length accordingly.
> 
> Parallel(Multi-CS):
> Both the flashes have their separate SPI bus, CS of both the flashes will be
> asserted/de-asserted at the same time. In this mode data will be split across both the
> flashes by enabling the STRIPE setting in the controller.
> Parallel mode is a controller feature where if the STRIPE bit is set then the controller
> internally handles the data split during data write to the flashes and while reading
> data from the flash the controller internally merges data from both the flashes before
> writing to the controller FIFO.
> If STRIPE is not enabled, then same data will be sent to both the devices.
> In parallel mode both the flashes should be identical.
> 
> For more information on the modes please feel free to go through the controller flash
> interface below [1].
> 
> Mirochip QSPI controller[2] also supports "Dual Parallel 8-bit IO mode", but they call
> it "Twin Quad Mode".
> 
> Initially in [3] [4] [5] Miquel had tried to extend MTD-CONCAT driver to support
> Stacked mode, but the bindings were not accepted. So, the MTD-CONCAT
> approach was dropped and the DT bindings that got accepted [6] [7] [8] that
> describes the two flash devices as being one. SPI core changes to support the
> above bindings were added [9]. While adding the support in SPI-NOR  Tudor
> provided additional feedback, leading to a discussion on updating the current
> stacked and parallel DT bindings.
> 
> Proposed Solution:
> The solution has two parts:
> 
> 1. Update MTD-CONCAT
>    Update MTD-CONCAT to create virtual concatinated mtd devices as defined
>    in the device tree.
> 
> 2. Add a New Layer
>    Add a new layer between the SPI-NOR and MTD layers to support stacked
>    and parallel configurations. This new layer will be part of spi-nor,
>    located in mtd/spi-nor/, can be included/excluded via Kconfig, will be
>    maintained by AMD and will:
> 
>    - During probing, store information from all connected flashes in
>      stacked or parallel mode and present them as a single device to the
> 	 MTD layer.
>    - Register callbacks and manage MTD device registration within the new
>      layer instead of spi-nor/core.c.
>    - Make minimal changes in spi-nor/core.c, as stacked and parallel
>      handling will be managed by the new layer on top of SPI-NOR.
>    - Handle odd byte count requests from the MTD layer during flash
>      operations in parallel mode.
> 
> [1] https://docs.amd.com/r/en-US/am011-versal-acap-trm/QSPI-Flash-Device-
> Interface
> [2]
> https://ww1.microchip.com/downloads/aemDocuments/documents/MPU32/ProductD
> ocuments/DataSheets/SAMA7G5-Series-Data-Sheet-DS60001765.pdf
> [3] https://lore.kernel.org/all/20191113171505.26128-4-miquel.raynal@bootlin.com/
> [4] https://lore.kernel.org/all/20191127105522.31445-5-miquel.raynal@bootlin.com/
> [5]https://lore.kernel.org/all/20211112152411.818321-1-miquel.raynal@bootlin.com/
> [6]
> https://github.com/torvalds/linux/commit/f89504300e94524d5d5846ff8b728592ac72c
> ec4
> [7]
> https://github.com/torvalds/linux/commit/eba5368503b4291db7819512600fa014ea17
> c5a8
> [8]
> https://github.com/torvalds/linux/commit/e2edd1b64f1c79e8abda365149ed62a2a9a4
> 94b4
> [9]https://github.com/torvalds/linux/commit/4d8ff6b0991d5e86b17b235fc46ec62e919
> 5cb9b
> 
> Thanks,
> Amit
> 
> Amit Kumar Mahapatra (2):
>   dt-bindings: mtd: Add bindings for describing concatinated MTD devices
>   dt-bindings: spi: Update stacked and parallel bindings
> 
>  .../mtd/partitions/fixed-partitions.yaml      | 18 +++++++++++++++
>  .../bindings/mtd/partitions/partitions.yaml   |  6 +++++
>  .../bindings/spi/spi-controller.yaml          | 23 +++++++++++++++++--
>  .../bindings/spi/spi-peripheral-props.yaml    |  9 +++-----
>  4 files changed, 48 insertions(+), 8 deletions(-)
> 
> --
> 2.34.1