Re: [PATCH V2 1/3] phy: Add binding document for Pistachio USB2.0 PHY

[Andrew Bresticker]

On Tue, May 5, 2015 at 4:35 PM, James Hogan <james.hogan@xxxxxxxxxx> wrote:
> On Tue, May 05, 2015 at 04:09:31PM -0700, Andrew Bresticker wrote:
>> On Tue, May 5, 2015 at 3:43 PM, James Hogan <james.hogan@xxxxxxxxxx> wrote:
>> > On Tue, May 05, 2015 at 03:16:23PM -0700, Andrew Bresticker wrote:
>> >> Hi James,
>> >>
>> >> On Tue, May 5, 2015 at 3:01 PM, James Hogan <james.hogan@xxxxxxxxxx> wrote:
>> >> > Hi Andrew,
>> >> >
>> >> > On Tue, Apr 07, 2015 at 03:04:16PM -0700, Andrew Bresticker wrote:
>> >> >> Add a binding document for the USB2.0 PHY found on the IMG Pistachio SoC.
>> >> >>
>> >> >> Signed-off-by: Andrew Bresticker <abrestic@xxxxxxxxxxxx>
>> >> >> Cc: Rob Herring <robh+dt@xxxxxxxxxx>
>> >> >> Cc: Pawel Moll <pawel.moll@xxxxxxx>
>> >> >> Cc: Mark Rutland <mark.rutland@xxxxxxx>
>> >> >> Cc: Ian Campbell <ijc+devicetree@xxxxxxxxxxxxxx>
>> >> >> Cc: Kumar Gala <galak@xxxxxxxxxxxxxx>
>> >> >> ---
>> >> >> No changes from v1.
>> >> >> ---
>> >> >>  .../devicetree/bindings/phy/pistachio-usb-phy.txt  | 29 ++++++++++++++++++++++
>> >> >>  include/dt-bindings/phy/phy-pistachio-usb.h        | 16 ++++++++++++
>> >> >>  2 files changed, 45 insertions(+)
>> >> >>  create mode 100644 Documentation/devicetree/bindings/phy/pistachio-usb-phy.txt
>> >> >>  create mode 100644 include/dt-bindings/phy/phy-pistachio-usb.h
>> >> >>
>> >> >> diff --git a/Documentation/devicetree/bindings/phy/pistachio-usb-phy.txt b/Documentation/devicetree/bindings/phy/pistachio-usb-phy.txt
>> >> >> new file mode 100644
>> >> >> index 0000000..afbc7e2
>> >> >> --- /dev/null
>> >> >> +++ b/Documentation/devicetree/bindings/phy/pistachio-usb-phy.txt
>> >> >> @@ -0,0 +1,29 @@
>> >> >> +IMG Pistachio USB PHY
>> >> >> +=====================
>> >> >> +
>> >> >> +Required properties:
>> >> >> +--------------------
>> >> >> + - compatible: Must be "img,pistachio-usb-phy".
>> >> >> + - #phy-cells: Must be 0.  See ./phy-bindings.txt for details.
>> >> >> + - clocks: Must contain an entry for each entry in clock-names.
>> >> >> +   See ../clock/clock-bindings.txt for details.
>> >> >> + - clock-names: Must include "usb_phy".
>> >> >> + - img,cr-top: Must constain a phandle to the CR_TOP syscon node.
>> >> >> + - img,refclk: Indicates the reference clock source for the USB PHY.
>> >> >> +   See <dt-bindings/phy/phy-pistachio-usb.h> for a list of valid values.
>> >> >
>> >> > Possibly dumb question: why isn't the reference clock source specified
>> >> > in the normal ways like the "usb_phy" clock is?
>> >> >
>> >> > Does the value required here depend on what usb_phy clock gets muxed
>> >> > from or something?
>> >>
>> >> Right, the value indicates what clock "usb_phy" is: whether it comes
>> >> from the core clock controller, the XO crystal, or is some external
>> >> clock.  It's a mux internal to the PHY.
>> >
>> > Okay. If its a software controllable mux, is there a particular reason
>> > the DT doesn't describe it as such, i.e. have all 3 clock inputs, and
>> > the driver somehow work out which to use?
>>
>> Well, I'm not sure how the driver would determine which clock to use
>> without a device-tree property like the one I've got here :).  Also,
>
> Does it make sense to just look for the "best" usable source clock based
> on the supported rates listed in fsel_rate_map[] (for some definition of
> "best" such as "fastest" / "slowest" / "first usable"), or are things
> just not that simple?
>
> I'm just wondering how the DT writer would decide, since it seems to
> come down to a policy decision rather than a description of the
> hardware, which should probably be avoided in DT bindings if possible.

Ah, sorry if that was unclear - this *is* describing a hardware
property.  The DT author would pick a value by looking at which clock
is connected to the PHY in the schematic.

-Andrew
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