linux-kernel - Re: [PATCHv2 1/3] dt-bindings: misc: achc: Make ezport distinguishable

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Date:   Thu, 26 Apr 2018 16:39:22 +0200
From:   Sebastian Reichel <sebastian.reichel@...labora.co.uk>
To:     Rob Herring <robh@...nel.org>, Mark Brown <broonie@...nel.org>
Cc:     linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        kernel@...labora.com
Subject: Re: [PATCHv2 1/3] dt-bindings: misc: achc: Make ezport
 distinguishable

Hi,

On Fri, Apr 13, 2018 at 10:33:05AM -0500, Rob Herring wrote:
> On Mon, Apr 9, 2018 at 4:13 PM, Sebastian Reichel
> <sebastian.reichel@...labora.co.uk> wrote:
> > Hi,
> >
> > On Mon, Apr 09, 2018 at 01:57:27PM -0500, Rob Herring wrote:
> >> On Tue, Mar 27, 2018 at 03:52:57PM +0200, Sebastian Reichel wrote:
> >> > This updates the GE ACHC binding, so that different compatible
> >> > strings are used for the programming interface, which is the
> >> > ezport interface from NXP MK20FN1M0VMD12 and the microcontroller's
> >> > normal SPI interface.
> >> >
> >> > Signed-off-by: Sebastian Reichel <sebastian.reichel@...labora.co.uk>
> >> > ---
> >> >  Documentation/devicetree/bindings/misc/ge-achc.txt | 19 ++++++++++++++++---
> >> >  1 file changed, 16 insertions(+), 3 deletions(-)
> >> >
> >> > diff --git a/Documentation/devicetree/bindings/misc/ge-achc.txt b/Documentation/devicetree/bindings/misc/ge-achc.txt
> >> > index 77df94d7a32f..6c6bd6568504 100644
> >> > --- a/Documentation/devicetree/bindings/misc/ge-achc.txt
> >> > +++ b/Documentation/devicetree/bindings/misc/ge-achc.txt
> >> > @@ -7,7 +7,13 @@ Note: This device does not expose the peripherals as USB devices.
> >> >
> >> >  Required properties:
> >> >
> >> > -- compatible : Should be "ge,achc"
> >> > +- compatible : Should be
> >> > +  "ge,achc" (normal interface)
> >> > +  "ge,achc-ezport" (flashing interface)
> >> > +
> >> > +Required properties (flashing interface only):
> >> > +
> >> > +- reset-gpios: GPIO Specifier for the reset GPIO
> >>
> >> Does the reset only affect the flashing interface and are the data pins
> >> shared? If not for both, then I think the correct thing to do here is
> >> just extend reg to support multiple values to represent multiple chip
> >> selects.
> >
> > reset affects the whole chip and the same spi data/clock pins are
> > being used, so extending reg should work. The flashing cannot happen
> > with the same speed, though. I'm currently encoding this by using
> > different "spi-max-frequency" properties. I suppose I could limit
> > it in the driver instead. I tried to come up with an example for
> > your suggestion. Is this what you had in mind?
> 
> If the max frequency is the device max, then that should be in the
> driver. spi-max-frequency should really only be needed if the
> frequency is less than the max of either the host or device.

Right.

> > &spi_controller {
> >     achc@...tiple {
> 
> @0
> 
> unit addresses are the 1st address.

Ok.

> >         /* 0 = flashing interface, 1 = normal interface */
> >         reg = <0>, <1>;
> 
> You may want to put the normal interface first as that is the primary
> interface and would still work assuming the OS ignored extra entries.
> 
> >         compatible = "ge,achc";
> >         reset-gpios = <&gpio42 23 ACTIVE_LOW>;
> >         spi-max-frequency = <42>; /* max speed for normal operation */
> 
> 42 Hz?

This was just a random number as example.

I had a look at implementing this and the Linux SPI core does not
expect any devices with multiple chip selects. I can try to
implement it, but I think it makes sense to gather some feedback
from Mark first (added to Cc).

@Mark
Patch Series: https://lwn.net/Articles/750177/
Binding Discussion: https://patchwork.kernel.org/patch/10310109/

-- Sebastian

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