[PATCH 1/3] usb: core: add power sequence for USB devices

[Rob Herring]

On Fri, Mar 04, 2016 at 10:37:50AM +0800, Peter Chen wrote:
> On Fri, Mar 04, 2016 at 03:23:05AM +0100, Andrew Lunn wrote:
> > On Fri, Mar 04, 2016 at 10:02:42AM +0800, Peter Chen wrote:
> > > On Thu, Mar 03, 2016 at 02:54:55PM -0600, Rob Herring wrote:
> > > > On Thu, Mar 3, 2016 at 4:01 AM, Peter Chen <peter.chen at nxp.com> wrote:
> > > > > Some hard-wired USB devices need to do power sequence to let the
> > > > > device work normally, the typical power sequence like: enable USB
> > > > > PHY clock, toggle reset pin, etc. But current Linux USB driver
> > > > > lacks of such code to do it, it may cause some hard-wired USB devices
> > > > > works abnormal or can't be recognized by controller at all.
> > > > >
> > > > > In this patch, it will do power on sequence at hub's probe for all
> > > > > devices under this hub (includes root hub) if this device is described
> > > > > at dts and there is a phandle "usb-pwrseq" for it. At hub_disconnect,
> > > > > it will do power off sequence.
> > > > >
> > > > > Signed-off-by: Peter Chen <peter.chen at nxp.com>
> > > > > ---
> > > > >  .../devicetree/bindings/usb/usb-device.txt         |  41 +++++-
> > > > >  drivers/usb/core/Makefile                          |   2 +-
> > > > >  drivers/usb/core/hub.c                             |  32 +++++
> > > > >  drivers/usb/core/pwrseq.c                          | 149 +++++++++++++++++++++
> > > > >  include/linux/usb/of.h                             |  10 ++
> > > > >  5 files changed, 232 insertions(+), 2 deletions(-)
> > > > >  create mode 100644 drivers/usb/core/pwrseq.c
> > > > >
> > > > > diff --git a/Documentation/devicetree/bindings/usb/usb-device.txt b/Documentation/devicetree/bindings/usb/usb-device.txt
> > > > > index 1c35e7b..c7a298c 100644
> > > > > --- a/Documentation/devicetree/bindings/usb/usb-device.txt
> > > > > +++ b/Documentation/devicetree/bindings/usb/usb-device.txt
> > > > > @@ -13,8 +13,37 @@ Required properties:
> > > > >  - reg: the port number which this device is connecting to, the range
> > > > >    is 1-31.
> > > > >
> > > > > +Optional properties:
> > > > > +- usb-pwrseq: the power sequence handler which need to do before this USB
> > > > > +  device can work.
> > > > > +- clocks: the input clock for USB device.
> > > > > +- clock-frequency: the frequency for device's clock.
> > > > > +- reset-gpios: Should specify the GPIO for reset.
> > > > > +- reset-duration-us: the duration in microsecond for assert reset signal.
> > > > 
> > > > So we sorted out how to describe USB devices in DT, but now we don't
> > > > use it?
> > > 
> > > We only know USB device after USB bus finds it, but without power
> > > on sequence, the USB bus can't find it.
> > 
> > Not really true. Device tree says it exists, you just cannot see it
> > yet. The fact it is in device tree means it is soldered on the board
> > and really is there. So when the host controller enumerates the bus,
> > it should run the power sequence of all child nodes to make them
> > appear.
> > 

Exactly.

> Yes, it is my patch doing. My words "We only know USB device after USB
> bus finds it" means the USB common code only create the USB device, and
> assign its .of_node after USB bus finds it.

I understand the problem. It is just as easy for you to search power 
sequence child nodes as searching the *actual* child nodes for devices. 
The main difference is that a power sequence node indicates that power 
sequencing can be handled generically. With the device nodes, you have 
to assume that the presence of standard properties like reset-gpios 
means you can handle it generically. 

Either solution suffers from not handling cases that can't be handled 
generically. No doubt, we'll just see continued extensions to keep 
things generic when they really shouldn't be.

Rob