Re: [PATCH v3 1/3] dt-bindings: mtd: spi-nor: Allow two CS per device

[<Tudor.Ambarus@microchip.com>]

Hi, Miquel,

On 12/6/21 11:59 AM, Miquel Raynal wrote:
> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> 
> The Xilinx QSPI controller has two advanced modes which allow the
> controller to behave differently and consider two flashes as one single
> storage.
> 
> One of these two modes is quite complex to support from a binding point
> of view and is the dual parallel memories. In this mode, each byte of
> data is stored in both devices: the even bits in one, the odd bits in
> the other. The split is automatically handled by the QSPI controller and
> is transparent for the user.
> 
> The other mode is simpler to support, it is called dual stacked
> memories. The controller shares the same SPI bus but each of the devices
> contain half of the data. Once in this mode, the controller does not
> follow CS requests but instead internally wires the two CS levels with
> the value of the most significant address bit.

The stacked mode that you describe seems particular to a specific
vendor. There are multi die NOR flashes which do not require any
controller intervention, the logic is held at the flash level:
https://media-www.micron.com/-/media/client/global/documents/products/technical-note/nor-flash/tn2505_n25q_mt25q_stacked_devices.pdf?rev=7a23cc95238e46f7b22e2a9f6bc736b7

Can you point us to which kind of memories you're willing to add
support for? Some datasheets will be best.

Cheers,
ta

> 
> Supporting these two modes will involve core changes which include the
> possibility of providing two CS for a single SPI device
> 
> Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
> Acked-by: Rob Herring <robh@kernel.org>
> ---
>  Documentation/devicetree/bindings/mtd/jedec,spi-nor.yaml | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/devicetree/bindings/mtd/jedec,spi-nor.yaml b/Documentation/devicetree/bindings/mtd/jedec,spi-nor.yaml
> index 39421f7233e4..4abfb4cfc157 100644
> --- a/Documentation/devicetree/bindings/mtd/jedec,spi-nor.yaml
> +++ b/Documentation/devicetree/bindings/mtd/jedec,spi-nor.yaml
> @@ -47,7 +47,8 @@ properties:
>        identified by the JEDEC READ ID opcode (0x9F).
> 
>    reg:
> -    maxItems: 1
> +    minItems: 1
> +    maxItems: 2
> 
>    spi-max-frequency: true
>    spi-rx-bus-width: true
> --
> 2.27.0
> 

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