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

[Rob Herring <robh+dt@xxxxxxxxxx>]

On Fri, Jun 1, 2018 at 12:09 PM, Boris Brezillon
<boris.brezillon@xxxxxxxxxxx> wrote:
> 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"

Seems awfully generic if you expect this alone. No chips with quirks
yet? Is there a standard for detection like jedec?

>> > +- reg: should encode the chip-select line used to access the NAND chip

"see spi.txt" should be enough.

>> > +
>> > +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.

This should always be the case. The operating frequency should be
min(host max, device max) unless the board has restrictions and needs
to set spi-max-frequency (and we really should have used
'bus-frequency' here). No doubt though, that is not what has been
done.

>> > +- 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.

Like frequency, this should have been similar. I imagine for the
common case, the number of host and device lines are 1 so it doesn't
really apply as this was added later on. Perhaps we can reword the
common definition to work for both?

Rob
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