Re: [PATCH v3 2/3] spi: dt-bindings: Describe stacked/parallel memories modes

[<Tudor.Ambarus@xxxxxxxxxxxxx>]

On 12/7/21 9:14 AM, Pratyush Yadav wrote:
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> 
> On 06/12/21 10:59AM, Miquel Raynal wrote:
>> Describe two new memories modes:
>> - A stacked mode when the bus is common but the address space extended
>>   with an additinals wires.
>> - A parallel mode with parallel busses accessing parallel flashes where
>>   the data is spread.
>>
>> Signed-off-by: Miquel Raynal <miquel.raynal@xxxxxxxxxxx>
>> ---
>>  .../bindings/spi/spi-peripheral-props.yaml    | 21 +++++++++++++++++++
>>  1 file changed, 21 insertions(+)
>>
>> diff --git a/Documentation/devicetree/bindings/spi/spi-peripheral-props.yaml b/Documentation/devicetree/bindings/spi/spi-peripheral-props.yaml
>> index 5dd209206e88..13aa6a2374c9 100644
>> --- a/Documentation/devicetree/bindings/spi/spi-peripheral-props.yaml
>> +++ b/Documentation/devicetree/bindings/spi/spi-peripheral-props.yaml
>> @@ -82,6 +82,27 @@ properties:
>>      description:
>>        Delay, in microseconds, after a write transfer.
>>
>> +  stacked-memories:
>> +    type: boolean
> 
> I don't think a boolean is enough to completely describe the memory.
> Sure, you say the memories are stacked, but where do you specify when to
> switch the CS? They could be two 512 MiB memories, two 1 GiB memories,
> or one 512 MiB and one 256 MiB.

If the multi-die flash contains identical dies then the die boundary can be
determined with flash_size / number_of_cs. Are there any multi die flashes
with different types of dies?

> 
>> +    description: Several SPI memories can be wired in stacked mode.
>> +      This basically means that either a device features several chip
>> +      selects, or that different devices must be seen as a single
>> +      bigger chip. This basically doubles (or more) the total address
>> +      space with only a single additional wire, while still needing
>> +      to repeat the commands when crossing a chip boundary. XIP is
>> +      usually not supported in this mode.
>> +
>> +  parallel-memories:
>> +    type: boolean
> 
> With this I assume both memories have to be the same size?

It looks like the assumption for both cases is that the dies are identical.

> 
>> +    description: Several SPI memories can be wired in parallel mode.
>> +      The devices are physically on a different buses but will always
>> +      act synchronously as each data word is spread across the
>> +      different memories (eg. even bits are stored in one memory, odd
>> +      bits in the other). This basically doubles the address space and
>> +      the throughput while greatly complexifying the wiring because as
>> +      many busses as devices must be wired. XIP is usually not
>> +      supported in this mode.
>> +
>>  # The controller specific properties go here.
>>  allOf:
>>    - $ref: cdns,qspi-nor-peripheral-props.yaml#
>> --
>> 2.27.0
>>
> 
> --
> Regards,
> Pratyush Yadav
> Texas Instruments Inc.
>