Re: [PATCH v5 2/3] spi: dt-bindings: Describe stacked/parallel memories modes

[Miquel Raynal <miquel.raynal@bootlin.com>]

Hi Tudor,

Tudor.Ambarus@microchip.com wrote on Wed, 22 Dec 2021 08:44:16 +0000:

> On 12/22/21 10:35 AM, Miquel Raynal wrote:
> > EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> > 
> > Hi Tudor,
> > 
> > Tudor.Ambarus@microchip.com wrote on Wed, 22 Dec 2021 08:22:05 +0000:  
> >> On 12/22/21 10:05 AM, Miquel Raynal wrote:  
> >>> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> >>>
> >>> Hello Tudor,  
> >>
> >> Hi!
> >>  
> >>>
> >>> Tudor.Ambarus@microchip.com wrote on Wed, 22 Dec 2021 07:52:44 +0000:
> >>>  
> >>>> On 12/21/21 7:00 PM, Miquel Raynal wrote:  
> >>>>> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> >>>>>
> >>>>> 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@bootlin.com>
> >>>>> ---
> >>>>>
> >>>>> Hello Rob,
> >>>>>
> >>>>> I know the below does not pass the tests (at least the example patch 3
> >>>>> does not pass) but I believe the issue is probably on the tooling side
> >>>>> because the exact same thing with uing32-array instead is accepted. The
> >>>>> problem comes from the minItems/maxItems lines. Without them, this is
> >>>>> okay. The maxItems btw matches the "good enough value for now" idea.
> >>>>>
> >>>>> The errors I get are:
> >>>>>
> >>>>> $ make dt_binding_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/spi/spi-controller.yaml
> >>>>>   LINT    Documentation/devicetree/bindings
> >>>>>   CHKDT   Documentation/devicetree/bindings/processed-schema-examples.json
> >>>>>   SCHEMA  Documentation/devicetree/bindings/processed-schema-examples.json
> >>>>>   DTEX    Documentation/devicetree/bindings/spi/spi-controller.example.dts
> >>>>>   DTC     Documentation/devicetree/bindings/spi/spi-controller.example.dt.yaml
> >>>>>   CHECK   Documentation/devicetree/bindings/spi/spi-controller.example.dt.yaml
> >>>>> /src/Documentation/devicetree/bindings/spi/spi-controller.example.dt.yaml: spi@80010000: flash@2:stacked-memories: [[268435456, 268435456]] is too short
> >>>>>         From schema: /src/Documentation/devicetree/bindings/spi/spi-controller.yaml
> >>>>> /src/Documentation/devicetree/bindings/spi/spi-controller.example.dt.yaml: spi@80010000: flash@2:stacked-memories: [[268435456, 268435456]] is too short
> >>>>>         From schema: /src/Documentation/devicetree/bindings/spi/mxs-spi.yaml
> >>>>> /src/Documentation/devicetree/bindings/spi/spi-controller.example.dt.yaml: spi@80010000: Unevaluated properties are not allowed ('#address-cells', '#size-cells', 'display@0', 'sensor@1', 'flash@2' were unexpected)
> >>>>>         From schema: /src/Documentation/devicetree/bindings/spi/mxs-spi.yaml
> >>>>> /src/Documentation/devicetree/bindings/spi/spi-controller.example.dt.yaml: flash@2: stacked-memories: [[268435456, 268435456]] is too short
> >>>>>         From schema: /src/Documentation/devicetree/bindings/mtd/jedec,spi-nor.yaml
> >>>>>
> >>>>>
> >>>>>  .../bindings/spi/spi-peripheral-props.yaml    | 25 +++++++++++++++++++
> >>>>>  1 file changed, 25 insertions(+)
> >>>>>
> >>>>> diff --git a/Documentation/devicetree/bindings/spi/spi-peripheral-props.yaml b/Documentation/devicetree/bindings/spi/spi-peripheral-props.yaml
> >>>>> index 5dd209206e88..fedb7ae98ff6 100644
> >>>>> --- a/Documentation/devicetree/bindings/spi/spi-peripheral-props.yaml
> >>>>> +++ b/Documentation/devicetree/bindings/spi/spi-peripheral-props.yaml
> >>>>> @@ -82,6 +82,31 @@ properties:
> >>>>>      description:
> >>>>>        Delay, in microseconds, after a write transfer.
> >>>>>
> >>>>> +  stacked-memories:
> >>>>> +    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. The size of
> >>>>> +      each chip should be provided as members of the array.
> >>>>> +    $ref: /schemas/types.yaml#/definitions/uint64-array
> >>>>> +    minItems: 2
> >>>>> +    maxItems: 4  
> >>>>
> >>>> Why do we define maxItems? Can't we remove this restriction?  
> >>>
> >>> Rob usually prefers to bound properties, that's why we often see "good
> >>> enough values for now" in the bindings. If it's no longer the case it's  
> >>
> >> right, I saw it.
> >>  
> >>> fine to drop the maxItems property.  
> >>
> >> There's no such hardware limitation as far as I know, that's why I've
> >> asked. Maybe Rob can advise.  
> > 
> > Yes, I'll follow what Rob thinks its best:
> > - keeping maxItems: 4 (as it is), which is a good enough value
> > - dropping the maxItems here because in the end no bounding is necessary  
> Then I would drop maxItems for stacked-memories. For parallel-memories:
> does the maxItems depend on the number of I/O lines?

Well, if you look into controller constraints, I bet most of them will
not support more than 8 data lines for now. For example, Xilinx QSPI
controller accepts up to two devices with 4 data lines on each. But I
believe it would be completely possible to use 4 devices with 2 data
lines or up to 8 with one as well. This is pure theory, I haven't
seen nor heard about such hosts so far, so let's wait for Rob advice on
that number and see what he has in mind.

> > - using maxItems: 2 to match the SPI CS even though in theory these two
> >   numbers are not correlated (stacked-memories might very well be
> >   used by other non SPI memories as well).
> > 
> > BTW if you're fine with the proposal your Ack is welcome ;)
> > 
> > Thanks,
> > Miquèl
> >   
> 

Thanks,
Miquèl