Date:	Wed, 22 Jun 2016 11:33:56 +0800
From:	Peter Chen <hzpeterchen@...il.com>
To:	Felipe Balbi <balbi@...nel.org>, yoshihiro.shimoda.uh@...esas.com
Cc:	Roger Quadros <rogerq@...com>, peter.chen@...escale.com,
	yoshihiro.shimoda.uh@...esas.com, tony@...mide.com,
	gregkh@...uxfoundation.org, dan.j.williams@...el.com,
	mathias.nyman@...ux.intel.com, Joao.Pinto@...opsys.com,
	sergei.shtylyov@...entembedded.com, jun.li@...escale.com,
	grygorii.strashko@...com, robh@...nel.org, nsekhar@...com,
	b-liu@...com, joe@...ches.com, linux-usb@...r.kernel.org,
	linux-omap@...r.kernel.org, linux-kernel@...r.kernel.org,
	devicetree@...r.kernel.org
Subject: Re: [PATCH v11 08/14] usb: otg: add OTG/dual-role core

On Tue, Jun 21, 2016 at 05:47:47PM +0300, Felipe Balbi wrote:
> 
> Hi,
> 
> Peter Chen <hzpeterchen@...il.com> writes:
> >> >> >> >> >>> + * @otg_dev: OTG controller device, if needs to be used with OTG core.
> >> >> >> >> >> 
> >> >> >> >> >> do you really know of any platform which has a separate OTG controller?
> >> >> >> >> >> 
> >> >> >> >> >
> >> >> >> >> > Andrew had pointed out in [1] that Tegra210 has separate blocks for OTG, host
> >> >> >> >> > and gadget.
> >> >> >> >> >
> >> >> >> >> > [1] http://article.gmane.org/gmane.linux.ports.tegra/22969
> >> >> >> >> 
> >> >> >> >> that's not an OTG controller, it's just a mux. No different than Intel's
> >> >> >> >> mux for swapping between XHCI and peripheral-only DWC3.
> >> >> >> >> 
> >> >> >> >> frankly, I would NEVER talk about OTG when type-C comes into play. They
> >> >> >> >> are two competing standards and, apparently, type-C is winning when it
> >> >> >> >> comes to role-swapping.
> >> >> >> >> 
> >> >> >> >
> >> >> >> > In fact, OTG is mis-used by people. Currently, if the port is dual-role,
> >> >> >> > It will be considered as an OTG port.
> >> >> >> 
> >> >> >> That's because "dual-role" is a non-standard OTG. Seen as people really
> >> >> >> didn't care about OTG, we (linux-usb folks) ended up naturally referring
> >> >> >> to "non-standard OTG" as "dual-role". Just to avoid confusion.
> >> >> >
> >> >> > So, unless we use OTG FSM defined in OTG spec, we should not mention
> >> >> > "OTG" in Linux, right?
> >> >> 
> >> >> to avoid confusion with the terminology, yes. With that settled, let's
> >> >> figure out how you can deliver what your marketting guys are asking of
> >> >> you.
> >> >> 
> >> >
> >> > Since nxp SoC claims they are OTG compliance, we need to pass usb.org
> >> > test. The internal bsp has passed PET test, and formal compliance test
> >> > is on the way (should pass too). 
> >> >
> >> > The dual-role and OTG compliance use the same zImage, but different
> >> > dtb.
> >> 
> >> okay, that's good to know. Now, the question really is: considering we
> >> only have one user for this generic OTG FSM layer, do we really need to
> >> make it generic at all? I mean, just look at how invasive a change that
> >> is.
> >
> > If the chipidea is the only user for this roger's framework, I don't
> > think it is necessary. In fact, Roger introduces this framework, and
> > the first user is dwc3, we think it can be used for others. Let's
> 
> Right, we need to look at the history of dwc3 to figure out why the
> conclusion that dwc3 needs this was made.
> 
> Roger started working on this framework when Power on Reset section of
> databook had some details which weren't always clear and, for safety, we
> always had reset asserted for a really long time. It was so long (about
> 400 ms) that resetting dwc3 for each role swap was just too much.
> 
> Coupled with that, the OTG chapter wasn't very clear either on
> expections from Host and Peripheral side initialization in OTG/DRD
> systems.
> 
> More recent version of dwc3 databook have a much better description of
> how and which reset bits _must_ be asserted and which shouldn't be
> touched unless it's for debugging purposes. When I implemented that, our
> ->probe() went from 400ms down to about 50us.
> 
> Coupled with that, the OTG chapter also became a lot clearer to the
> point that it states you just don't initialize anything other than the
> OTG block, and just wait for OTG interrupt to do whatever it is you need
> to do.
> 
> This meant that we could actually afford to do full reinitialization of
> dwc3 on role swap (it's now only 50us anyway) and we knew how to swap
> roles properly.
> 
> (The reason for needing soft-reset during role swap is kinda long. But
> in summary dwc3 shadows register writes to both host and peripheral
> sides)
> 
> > just discuss if it is necessary for dual-role switch.
> 
> fair. However, if we have a single user we don't have a Generic
> layer. There's not enough variance to come up with truly generic
> architecture for this.
> 
> -- 

I have put some points in my last reply [1], I summery it here to
see if a generic framework is deserved or not?

1. If there are some parts we can use during the role switch
- The common start/stop host and peripheral operation
eg, when switch from host to peripheral, all drivers can use
usb_remove_hcd to finish it.
- A common workqueue to handle vbus and id event
- sysfs for role switch

2. Does a mux driver can do it well? Yoshihiro, here we need your
point. The main point is if we need to call USB API to change
roles (eg, usb_remove_hcd) during the role switch, thanks.


[1] http://www.spinics.net/lists/linux-usb/msg142974.html
-- 

Best Regards,
Peter Chen

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