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

[Pratyush Yadav]

On 07/12/21 07:35AM, Tudor.Ambarus at microchip.com wrote:
> On 12/7/21 9:14 AM, Pratyush Yadav wrote:
> > EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> > 
> > 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 at bootlin.com>
> >> ---
> >>  .../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?

The way I see it, a multi-die flash is not much different from 2 
independent flashes attached to the same SPI bus. So if we are going to 
implement this feature, I want it to be generic enough to allow 
supporting this type of hardware setup as well.

I am not aware of any flashes with a different CS for each die (that 
isn't handled by the flash internally), let alone with different types 
of dies. IIRC from our IRC conversation, Miquel's use case was using 2 
smaller identical flashes connected to the same SPI bus with 1 CS each. 
Do I remember this right Miquel?

> 
> > 
> >> +    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.

I would like to _not_ assume that for stacked-memories, unless 
implementing that becomes too complicated.

> 
> > 
> >> +    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.
> > 
> 

-- 
Regards,
Pratyush Yadav
Texas Instruments Inc.