Hi Roger,

On 2017-01-19 17:45, Roger Quadros wrote:

Vivek,

On 19/01/17 13:56, Vivek Gautam wrote:

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Hi,


On Wed, Jun 22, 2016 at 2:00 PM, Roger Quadros [off-list ref] wrote:

Luckily hit this thread while checking about DRD role functionality 
for DWC3.

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On 22/06/16 11:14, Felipe Balbi wrote:

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Hi,

Roger Quadros [off-list ref] writes:

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For the real use case, some Carplay platforms need it.

Carplay does *NOT* rely on OTG. Apple has its own 
proprietary and closed
specification which is not OTG-compliant.

Yes, it is not OTG-compliant, but it can co-work with some 
standard OTG FSM
states to finish role swap.

What are you referring to as "finish role swap"? I don't get 
that.

Change current role from host to peripheral.

Okay, we have two scenarios here:

1. You need full OTG compliance

  For this, granted, you need the state machine if your HW 
doesn't
  track it. This is a given. With only one user, however, 
perhaps
  we don't need a generic layer. There are not enough different
  setups to design a good enough generic layer. We will end up
  with a pseudo-generic framework which is coupled with its only
  user.

2. Dual-role support, without OTG compliance

  In this case, you don't need a stack. All you need is a signal
  to tell you state of ID pin and another to tell you state of
  VBUS level. If you have those, you don't need to walk an OTG
  state machine at all. You don't need any of those quirky OTG
  timers, agreed?

  Given the above, why would you even want to use a subset of 
OTG
  state machine to implement something that's _usually_ as 
simple
  as:

8<----------------------------------------------------------------------
  vbus = read(VBUS_STATE); /* could be a gpio_get_value() */
        id = read(ID_STATE); /* could be a gpio_get_value() */

        set_role(id);
        set_vbus(vbus);
------------------------------------------------------------------------

In fact, the individual driver can do it by itself. The chipidea 
driver
handles OTG and dual-role well currently. By considering this 
OTG/DRD
framework is worthwhile or not, we would like to see if it can
simplify DRD design for each driver, and can benefit the 
platforms which
has different drivers for host and peripheral to finish the role 
switch
well.

simplify how?  By adding unnecessary workqueues and a level 
indirection
that just goes back to the same driver?

What do you mean by same driver?

dwc3 registers to OTG layer. dwc3 also registers as UDC to UDC
layer. When dwc3 OTG IRQ fires, dwc3 tells OTG layer about it and 
OTG
layer jumps to a callback that goes back to dwc3 to e.g. start
peripheral side.

See ?!? Starts on dwc3, goes to OTG layer, goes back to DWC3.

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Gadget driver, host driver and PHY (or MUX) driver (for ID/VBUS) 
can
be 3 totally independent drivers unlike dwc3 where you have a 
single
driver in control of both host and gadget.

That's a totally different issue and one not being tackled by OTG
layer, because there are no such users yet. We can't design anything
based solely on speculation of what might happen.

If there aren't enough users, there is no way to design a good 
generic
layer.

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Questions not clear to me are:

1) Which driver handles ID/VBUS events and makes a decision to do 
the
role swap?  Probably the PHY/MUX driver?

This is implementation dependent. For TI's USB subsystem, we have 
PMIC
sampling VBUS/ID that and using EXTCON to tell dwc3-omap to program 
UTMI
mailbox. The same mailbox can be used in HW-mode (see AM437x) where 
SW
has no intervention.

For Intel's USB subsystem, we have PMIC sampling VBUS/ID with an
internal mux (much like TI's UTMI mailbox, but slightly different) 
to
switch between a separate XHCI or a separate dwc3. The same mux can 
be
put in HW-mode where SW has no intervention.

In any case, for Intel's stuff most of the magic happens in ASL. Our 
PHY
driver just detects role (at least for Type-C based plats) and 
executes
_DSM with correct arguments [1]. _DSM will program internal MUX, 
toggle
VBUS and, for type-C, toggle VCONN when needed.

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2) How does it perform the role swap? Probably a register write to 
the
PHY/MUX without needing to stop/start controllers? Easy case is 
both
controllers can run in co-existence without interference. Is there 
any
platform other than dwc3 where this is not the case?

Again speculation. But to answer your question, only dwc3 is in such 
a
case today. But even for dwc3 we can have DRD with a much, much 
simpler
setup as I have already explained.

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3) Even if host and gadget controllers can operate in coexistence,
there is no need for both to be running for embedded applications
which are usually power conservative.  How can we achieve that?

Now you're also speculating that you're running on embedded 
applications
and that we _can_ power off parts of the IP. I happen to know that 
we
can't power off XHCI part of dwc3 in TI's SoC because that's fed by 
same
Clocks and power rails as the peripheral side.

[1] https://lkml.org/lkml/2016/6/21/658

For TI's case it is dwc3 and you are implementing the role swap in 
the dwc3
driver where you do intend to remove the XHCI platform device. So I'm 
not
much concerned about that.

I was concerned about other platforms. I guess I'll let the other 
platform
people speak up as to what they need.

I will talk about the msm platforms using dwc3 hardware.
DWC3 controller on msm doesn't seem to have full otg functionality,
and the driver makes use of switching between host and device
using PRTCAPDIR register in of the core [1].
test
We plan to support this DRD role switching (swapping host and device
functionality based on id/vbus interrupts) in upstream.

Do we see a valid case to have this framework?

Felipe wanted to have a minimal dual-role logic inside dwc3 which is
independent of any DRD/OTG framework.

I have implemented this and will send out patches today for review.

Okay, good to know that. I will be happy to take a look at the
patches and test them for msm.

Thanks for sharing the info.

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Or, may be add a 'drd' layer for dwc3 that handles
role switching (using PRTCAPDIR) based on the id/vbus extcon 
notifications.


[1] 
https://source.codeaurora.org/quic/la/kernel/msm-3.18/tree/drivers/usb/dwc3/dwc3-msm.c?h=msm-3.18
     "dwc3_otg_start_host()"
     "dwc3_otg_start_peripheral()"

regards,
-roger

Regards
Vivek