Re: [PATCH v5 1/3] dt-bindings: mtd: spi-nor: Allow two CS per device

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

miquel.raynal@bootlin.com wrote on Wed, 22 Dec 2021 09:23:24 +0100:

> Hi Pratyush,
> 
> p.yadav@ti.com wrote on Wed, 22 Dec 2021 00:17:27 +0530:
> 
> > On 21/12/21 06:00PM, Miquel Raynal wrote:  
> > > 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.
> > > 
> > > 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    
> > 
> > You allow up to 4 items in stacked-memories but only allow up to 2 CS, 
> > which would make the other 2 memories unusable. Should also change this 
> > to 4.  
> 
> Yes, I allowed "more" theoretical devices in the
> stacked/parallel-memories properties because there is no real
> limitation on this side so I didn't want to constrain it too much,
> while still keeping a maximum value, hence 4 seemed a nice guess for a
> "maximum but can be bigger value we don't really care it's just for
> bounding". However on the SPI side this is a big change with deep
> consequences and I don't want to rush things so it is on purpose that I
> kept the limitation to 2. But we can change the maxItems to 2
> everywhere if this appears to be the thing to do.

I forgot to mention that the stacked/parallel-memories could
also certainly be considered "memory" properties (think about the
generic term), not necessarily bound to SPI. We could definitely have
the same pattern with other memory types as well and not be tight to
the number of SPI CS.

Thanks,
Miquèl