On Thu, May 11, 2017 at 2:12 PM, Michael Heimpold [off-list ref] wrote:

Hi,

Am Mittwoch, 10. Mai 2017, 10:53:26 CEST schrieb Stefan Wahren:

quoted

This merges the serdev binding for the QCA7000 UART driver (Ethernet over
UART) into the existing document.

Signed-off-by: Stefan Wahren <redacted>
---
 .../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.

In general this wouldn't work if the default SPI mode is used. Then
maybe you rely on reg property instead. But that all seems a little
fragile to me.

(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)

It's the other way around. The parent controls init/probing of the
children. Maybe another OS could do things differently I suppose.

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 :-)

It's true that we don't really need the interface as part of the
compatible, but I don't think it really hurts anything to have
something more specific. The firmware is also part of the hardware, so
a compatible that also implies firmware operation is not uncommon. I'm
fine either way.

Rob