linux-kernel - Re: [PATCH] USB: core: Add warm reset while reset-resuming SuperSpeed HUBs

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Date:	Fri, 13 Dec 2013 10:27:14 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Sarah Sharp <sarah.a.sharp@...ux.intel.com>,
	Julius Werner <jwerner@...omium.org>,
	Vikas Sajjan <sajjan.linux@...il.com>,
	Vikas Sajjan <vikas.sajjan@...aro.org>,
	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
	Kukjin Kim <kgene.kim@...sung.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	Vincent Palatin <vpalatin@...omium.org>,
	Lan Tianyu <tianyu.lan@...el.com>,
	Ksenia Ragiadakou <burzalodowa@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Vivek Gautam <gautam.vivek@...sung.com>,
	Douglas Anderson <dianders@...omium.org>,
	Felipe Balbi <balbi@...com>, sunil joshi <joshi@...sung.com>
Subject: Re: [PATCH] USB: core: Add warm reset while reset-resuming SuperSpeed HUBs

On Fri, Dec 13, 2013 at 10:15 AM, Alan Stern <stern@...land.harvard.edu> wrote:
> On Fri, 13 Dec 2013, Dan Williams wrote:
>
>> > I'm actually leaning towards enabling the check for warm reset broadly.
>> > It seems like it wouldn't hurt to issue a warm reset on the USB 3.0
>> > ports if they're in compliance, poll, or rx.detect.  So, let's enable
>> > this broadly in xhci_resume, mark the patch for stable, but ask for the
>> > backport to be delayed until 3.13.3 is out, to allow for more testing.
>> > If anyone complains of xHCI behavior changes, we'll change the code to
>> > add a quirk.
>>
>> Is there a clean way to make this per-port rather than globally at
>> xhci_resume()?  I am looking to hook into this for port power recovery
>> as Tianyu's testing encountered "warm reset required" conditions at
>> runtime_resume.  I'm still on the hunt for a solid reproducer, but it
>> indicates this is a more general quirk with power session recovery.
>
> There's no reason you can't do per-port testing inside xhci_resume
> (assuming you know what to test for) as well as putting a warm reset in
> the port-power handler of xhci_hub_control.

I'm just uneasy putting the recovery there as we lose the context of
why the port was powered-on.  For example we don't want to
pre-empt/duplicate a reset in xhci_hub_control() that is already
specified in hub_events().

> Of course, doing simultaneous warm resets on multiple ports will use
> less time than resetting each port individually, in sequence.
>

For the hub resume case, yes.  For pm_runtime_resume of an individual
port I believe it needs to be synchronous.
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