linux-kernel - Re: [PATCH v2 4/4] usb: dwc3: Workaround for super-speed host on dra7 in dual-role mode

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Date:   Fri, 31 Mar 2017 10:43:39 +0300
From:   Roger Quadros <rogerq@...com>
To:     Felipe Balbi <balbi@...nel.org>
CC:     <vivek.gautam@...eaurora.org>, <linux-usb@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 4/4] usb: dwc3: Workaround for super-speed host on dra7
 in dual-role mode

Hi,

On 29/03/17 13:32, Felipe Balbi wrote:
> 
> Hi,
> 
> Roger Quadros <rogerq@...com> writes:
>>> Roger Quadros <rogerq@...com> writes:
>>>> dra7 OTG core limits the host controller to USB2.0 (high-speed) mode
>>>> when we're operating in dual-role.
>>>
>>> yeah, that's not a quirk. DRA7 supports OTGv2, not OTGv3. There was no
>>> USB3 when OTGv2 was written.
>>>
>>> DRA7 just shouldn't use OTG core altogether. In fact, this is the very
>>> thing I've been saying for a long time. Make the simplest implementation
>>> possible. The dead simple, does-one-thing-only sort of implementation.
>>>
>>> All we need for Dual-Role (without OTG extras) is some input for ID and
>>> VBUS, then we add/remove HCD/UDC conditionally and set PRTCAPDIR.
>>>
>>
>> The catch is that on AM437x there is no way to get ID and VBUS events other
>> than the OTG controller so we have to rely on the OTG controller for that. :(
> 
> okay, so AM437x can get OTG interrupts properly. That's fine. We can
> still do everything we need using code that's already existing in dwc3
> if we refactor it a bit and hook it up to the OTG IRQ handler.
> 
> Here's what we do:
> 
> * First we re-factor all necessary code around so the API for OTG/DRD
>   is resumed to calling:
> 
> 	dwc3_add_udc(dwc);
>         dwc3_del_udc(dwc);
>         dwc3_add_hcd(dwc);
>         dwc3_del_hcd(dwc);

Why do we need these new APIs? don't these suffice?
	dwc3_gadget_init(dwc);
	dwc3_gadget_exit(dwc);
	dwc3_host_init(dwc);
	dwc3_host_exit(dwc);
> 
> the semantics of these should be easy to understand and you can
> implement each in their respective host.c/gadget.c files.
> 
> * Second step is to modify our dwc3_init_mode() (or whatever that
>   function was called, sorry, didn't check) to make sure we have
>   something like:
> 
> 	case OTG:
>         	dwc3_add_udc(dwc);
>                 break;
> 
> We should *not* add HCD in this case yet.
> 
> * After that we add otg.c (or drd.c, no preference) and make that call
>   dwc3_add_udc(dwc) and, also, provide
>   dwc3_add_otg(dwc)/dwc3_del_otg(dwc) calls. Then patch the switch
>   statement above to:
> 
> 	case OTG:
>         	dwc3_add_otg(dwc);
>                 break;
> 
> Note that at this point, this is simply a direct replacement of
> dwc3_add_udc() to dwc3_add_otg(). This should maintain current behavior
> (which is starting with peripheral mode by default), but it should also
> add support for OTG interrupts to change the mode (from an interrupt
> thead)
> 
> 	otg_isr()
>         {
> 
> 		/* don't forget to remove preivous mode if necessary */
>         	if (perimode)
>                 	dwc3_add_udc(dwc);
>                 else
>                 	dwc3_add_hcd(dwc);
> 	}
> 
> * The next patch would be to choose default conditionally based on
>   PERIMODE or whatever.
> 
> Of course, this is an oversimplified view of reality. You still need to
> poke around at PRTCAPDIR, etc. But all this can, actually, be prototyped
> using our "mode" debugfs file. Just make that call
> dwc3_add/del_udc/hcd() apart from fiddling with PRTCAPDIR in GCTL.

We also need to ensure that system suspend/resume doesn't break.
Mainly if we suspend/resume with UDC removed.
> 
> Your first implementation could be just that. Refactoring what needs to
> be refactored, then patching "mode" debugfs to work properly in that
> case. Only add otg.c/drd.c after "mode" debugfs file is stable, because
> then you know what needs to be taken into consideration.
> 
> Just to be clear, I'm not saying we should *ONLY* get the debugfs
> interface for v4.12, I'm saying you should start with that and get that
> stable and working properly (make an infinite loop constantly changing
> modes and keep it running over the weekend) before you add support for
> OTG interrupts, which could come in the same series ;-)
> 

Just to clarify debugfs mode behaviour.

Currently it is just changing PRTCAPDIR. What we need to do is that if
dr_mode == "otg", then we call dwc3_host/gadget_init/exit() accordingly as well.

Does this make sense?

cheers,
-roger