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Date:   Thu, 11 May 2017 23:43:42 -0700
From:   Jakub Kicinski <kubakici@...pl>
To:     Michael Heimpold <mhei@...mpold.de>
Cc:     Rob Herring <robh@...nel.org>,
        Stefan Wahren <stefan.wahren@...e.com>,
        Mark Rutland <mark.rutland@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        devicetree@...r.kernel.org
Subject: Re: [PATCH v5 15/17] dt-bindings: qca7000: append UART interface to
 binding

On Fri, 12 May 2017 06:15:52 +0000, Michael Heimpold wrote:
> Hi,
> 
> Zitat von Jakub Kicinski <kubakici@...pl>:
> 
> > 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@...e.com>
> >> > ---
> >> >  .../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@...14000 {
> >> >  		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.

I think drivers/tty/serial/sc16is7xx.c has the same strings, and some
Kconfig magic to work when either bus is enabled in .config.

Quick grep shows there are couple more potential ones to look at:

$ find . -name Kconfig | xargs grep -n 'SPI_MASTER.* I2C' 
./drivers/tty/serial/Kconfig:1208:        depends on (SPI_MASTER && !I2C) || I2C
./drivers/mfd/Kconfig:327:	depends on (SPI_MASTER || I2C)
./drivers/iio/dac/Kconfig:10:	depends on (SPI_MASTER && I2C!=m) || I2C
./drivers/iio/dac/Kconfig:34:	depends on (SPI_MASTER && I2C!=m) || I2C
./drivers/iio/dac/Kconfig:57:	depends on (SPI_MASTER && I2C!=m) || I2C
./drivers/gpio/Kconfig:1231:	depends on (SPI_MASTER && !I2C) || I2C
$ find . -name Kconfig | xargs grep -n 'I2C.*||.*SPI_MASTER' 
./drivers/mfd/Kconfig:1094:	depends on (I2C=y || SPI_MASTER=y)
./drivers/iio/gyro/Kconfig:55:	depends on (I2C || SPI_MASTER)
./drivers/iio/gyro/Kconfig:107:	depends on (I2C || SPI_MASTER) && SYSFS
./drivers/iio/accel/Kconfig:153:	depends on (I2C || SPI_MASTER) && SYSFS
./drivers/iio/pressure/Kconfig:20:	depends on (I2C || SPI_MASTER)
./drivers/iio/pressure/Kconfig:161:	depends on (I2C || SPI_MASTER) && SYSFS
./drivers/iio/magnetometer/Kconfig:118:	depends on (I2C || SPI_MASTER) && SYSFS

drivers/mfd/mc13xxx-*.c seems to have the same strings.  The iio/dac drivers
don't support DT but do share names.  The MCP GPIO chip you mention indeed has
different product names based on the bus it's made for (0 vs s in the middle 
of the name), so I gather less relevant case?  drivers/iio/pressure/bmp280-*.c 
has the same strings, if I'm looking correctly... I didn't look at the others.

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