Re: [PATCH v5 15/17] dt-bindings: qca7000: append UART interface to binding

[Michael Heimpold]

Hi,

Zitat von Jakub Kicinski <kubakici@xxxxx>:

> On Thu, 11 May 2017 21:12:22 +0200, Michael Heimpold wrote:
> > Am Mittwoch, 10. Mai 2017, 10:53:26 CEST schrieb Stefan Wahren:
> > > This merges the serdev binding for the QCA7000 UART driver (Ethernet over
> > > UART) into the existing document.
> > >
> > > Signed-off-by: Stefan Wahren <stefan.wahren@xxxxxxxx>
> > > ---
> > >  .../devicetree/bindings/net/qca-qca7000.txt        | 32
> > > ++++++++++++++++++++++ 1 file changed, 32 insertions(+)
> > >
> > > diff --git a/Documentation/devicetree/bindings/net/qca-qca7000.txt
> > > b/Documentation/devicetree/bindings/net/qca-qca7000.txt index
> > > a37f656..08364c3 100644
> > > --- a/Documentation/devicetree/bindings/net/qca-qca7000.txt
> > > +++ b/Documentation/devicetree/bindings/net/qca-qca7000.txt
> > > @@ -54,3 +54,35 @@ ssp2: spi@80014000 {
> > >  		local-mac-address = [ A0 B0 C0 D0 E0 F0 ];
> > >  	};
> > >  };
> > > +
> > > +(b) Ethernet over UART
> > > +
> > > +In order to use the QCA7000 as UART slave it must be defined as  
> > a child of
> > > a +UART master in the device tree. It is possible to preconfigure the UART
> > > +settings of the QCA7000 firmware, but it's not possible to change them
> > > during +runtime.
> > > +
> > > +Required properties:
> > > +- compatible        : Should be "qca,qca7000-uart"
> >
> > I already discussed this with Stefan off-list a little bit, but I would like
> > to bring this to a broader audience: I'm not sure whether the compatible
> > should contain the "-uart" suffix, because the hardware chip is the  
> > very same
> > QCA7000 chip which can also be used with SPI protocol.
> > The only difference is the loaded firmware within the chip which can either
> > speak SPI or UART protocol (but not both at the same time - due to shared
> > pins). So the hardware design decides which interface type is used.
> >
> > At the moment, this patch series adds a dedicated driver for the UART
> > protocol, in parallel to the existing SPI driver. So a different compatible
> > string is needed here to match against the new driver.
> >
> > An alternative approach would be to re-use the existing compatible string
> > "qca,qca7000" for both, the SPI and UART protocol, because a "smarter"
> > (combined) driver would detect which protocol to use. For example the driver
> > could check for spi-cpha and/or spi-cpol which are required for SPI  
> > protocol:
> > if these exists the driver could assume that SPI must be used, if both are
> > missing then UART protocol should be used.
> > (This way it would not be necessary to check whether the node is a child of
> > a SPI or UART master node - but maybe this is even easier - I don't know)
> >
> > Or in shorter words: my concern is that while "qca7000-uart" describes the
> > hardware, it's too closely coupled to the driver implementation. Having
> > some feedback of the experts would be nice :-)
>
> I'm no expert, but devices which can do both I2C and SPI are quite
> common, and they usually have the same compatible string for both
> buses.

do you have an example driver at hand? I only found GPIO mcp23s08 driver,
which can handle both I2C and SPI chips, but there are different compatible
strings used to distinguish several chip models.

Regards,
Michael