linux-kernel - Re: [PATCH V4] usb: remove unnecessary CONFIG_PM dependency from USB

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Message-ID: <87r3k6mngw.fsf@saruman.tx.rr.com>
Date:	Tue, 3 Nov 2015 21:51:11 -0600
From:	Felipe Balbi <balbi@...com>
To:	Peter Chen <peter.chen@...escale.com>
CC:	Nathan Sullivan <nathan.sullivan@...com>,
	<gregkh@...uxfoundation.org>, <linux-usb@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V4] usb: remove unnecessary CONFIG_PM dependency from USB_OTG


Hi,

Peter Chen <peter.chen@...escale.com> writes:
> On Tue, Nov 03, 2015 at 07:56:55AM -0600, Felipe Balbi wrote:
>> 
>> Hi,
>> 
>> Nathan Sullivan <nathan.sullivan@...com> writes:
>> > The USB OTG support currently depends on power management
>> > (CONFIG_PM) being enabled, but does not actually need it enabled.
>> > Remove this dependency.
>> >
>> > Tested on Bay Trail hardware with dwc3 USB.
>> >
>> > Signed-off-by: Nathan Sullivan <nathan.sullivan@...com>
>> > ---
>> >  drivers/usb/core/Kconfig |    1 -
>> >  1 file changed, 1 deletion(-)
>> >
>> > diff --git a/drivers/usb/core/Kconfig b/drivers/usb/core/Kconfig
>> > index a99c89e..9c5cdf3 100644
>> > --- a/drivers/usb/core/Kconfig
>> > +++ b/drivers/usb/core/Kconfig
>> > @@ -43,7 +43,6 @@ config USB_DYNAMIC_MINORS
>> >  
>> >  config USB_OTG
>> >  	bool "OTG support"
>> > -	depends on PM
>> 
>> I don't think this is correct. OTG depends on USB bus suspend, which is
>> only available on PM builds. Care to further detail why you think PM is
>> not needed on OTG ?
>> 
>
> OTG depends on USB bus suspend is not a must, the hardware controlled OTG
> design do HNP when the bus goes to suspend; but if the software
> implements OTG FSM, it is the user option whether do HNP, and bus
> suspend is controlled by OTG FSM software (stop SOF), but not by host 
> stack (eg, ehci).
>
> I am sorry I did not consider the legacy OTG design, this patch should
> be dropped.

there is no "legacy" OTG design. OTG requires a bus suspend to enter
HNP, and that's achieved by stopping all transfers and avoid new URB
submission so usbcore can put the bus in suspend (by means of USB
autosuspend). If you're bypassing that in the OTG FSM thing, that needs
to be fixed ASAP as that makes it a lot harder for any generic changes
in usbcore to be validated. Specially when you consider not many will
have whatever special HW which, likely, doesn't even work with mainline
to validate a change.

Please, make sure to fix that design so that HNP *always* goes through
the proper code path. If you have devices which would prevent HNP
because their class driver (host side driver) would never autosuspend,
fix that as well.

cheers

-- 
balbi

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