Re: [PATCH v8 2/4] dt-bindings: Add bindings for SPI NAND devices

[Boris Brezillon <boris.brezillon@xxxxxxxxxxx>]

Hi Geert,

On Fri, 1 Jun 2018 16:42:26 +0200
Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:

> Hi Boris,
> 
> I became interested after reading the cover letter...
> 
> On Fri, Jun 1, 2018 at 3:13 PM, Boris Brezillon
> <boris.brezillon@xxxxxxxxxxx> wrote:
> > Add bindings for SPI NAND chips.
> >
> > Signed-off-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxx>
> 
> Thanks for your patch!

And thanks for reviewing it ;-).

> 
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/mtd/spi-nand.txt
> > @@ -0,0 +1,27 @@
> > +SPI NAND flash
> > +
> > +Required properties:
> > +- compatible: should be "spi-nand"
> > +- reg: should encode the chip-select line used to access the NAND chip
> > +
> > +Optional properties
> > +- spi-max-frequency: maximum frequency of the SPI bus the chip can operate at.
> > +                    This should encode board limitations (i.e. max freq can't
> > +                    be achieved due to crosstalk on IO lines).
> > +                    When unspecified, the driver assumes the chip can run at
> > +                    the max frequency defined in the spec (information
> > +                    extracted chip detection time).
> 
> This is a standard property according to
> Documentation/devicetree/bindings/spi/spi-bus.txt. Can't you just refer
> to that file, or just omit it, as it applies to all SPI slaves anyway?

The thing is, the maximum frequency supported by a SPI NAND is directly
encoded in the NAND device ID and can be auto-detected. Why should we
define spi-max-frequency in the DT when we can automatically detect
this information? The only reason one might want to override
spi-max-frequency is when the board design impose such restrictions,
hence the precision I give here. 

> 
> > +- spi-tx-bus-width: The bus width (number of data wires) that is used for MOSI.
> > +                   Only encodes the board constraints (i.e. when not all IO
> > +                   signals are routed on the board). Device constraints are
> > +                   extracted when detecting the chip, and controller
> > +                   constraints are exposed by the SPI mem controller. If this
> > +                   property is missing that means no constraint at the board
> > +                   level.
> > +- spi-rx-bus-width: The bus width (number of data wires) that is used for MISO.
> > +                   Only encodes the board constraints (i.e. when not all IO
> > +                   signals are routed on the board). Device constraints are
> > +                   extracted when detecting the chip, and controller
> > +                   constraints are exposed by the SPI mem controller. If this
> > +                   property is missing that means no constraint at the board
> > +                   level.
> 
> This does not match Documentation/devicetree/bindings/spi/spi-bus.txt,
> which says the default is 1.

Yes, I know.

> 
> As these properties are handled by the SPI core in of_spi_parse_dt, why
> would you want to deviate?

Because, again, this information can be extracted from the NAND ID, and
the only reason we might want to override the information extracted
from the NAND ID is when the board design adds extra restrictions
(like, only 2 SIO lines wired on the 4 available).

> 
> Commenting to the question in the cover letter: what would be the
> purpose of spi-max-bus-width?

Defining how many IO lines are wired on the board design. If this
property is missing we would assume all IO lines are wired and the
restrictions would be negotiated between the controller and the device
without requiring explicit description in the DT.

Regards,

Boris 
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