Re: [PATCH 0/2] usb: override hub device bInterval with device node

From: Alan Stern <stern@rowland.harvard.edu>
To: Ikjoon Jang <ikjn@chromium.org>
Cc: linux-usb@vger.kernel.org, <devicetree@vger.kernel.org>,
	<linux-kernel@vger.kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Rob Herring <robh+dt@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Suwan Kim <suwan.kim027@gmail.com>,
	"Gustavo A . R . Silva" <gustavo@embeddedor.com>,
	Johan Hovold <johan@kernel.org>,
	Nicolas Boitchat <drinkcat@chromium.org>
Subject: Re: [PATCH 0/2] usb: override hub device bInterval with device node
Date: Tue, 19 Nov 2019 10:14:27 -0500 (EST)	[thread overview]
Message-ID: <Pine.LNX.4.44L0.1911191008440.1506-100000@iolanthe.rowland.org> (raw)
In-Reply-To: <CAATdQgBPrk=obCOiMAe1zAoP1As21MuzGzn-ixU56EmSkdQr1w@mail.gmail.com>

On Tue, 19 Nov 2019, Ikjoon Jang wrote:

> On Sun, Nov 17, 2019 at 11:46 PM Alan Stern <stern@rowland.harvard.edu> wrote:
> >
> > On Sun, 17 Nov 2019, Ikjoon Jang wrote:
> >
> > > This patchset enables hard wired hub device to use different bInterval
> > > from its descriptor when the hub has a combined device node.
> > >
> > > When we know the specific hard wired hub supports changing its polling
> > > interval, we can adjust hub's interval to reduce the time of waking up
> > > from autosuspend or connect detection of HIDs.
> >
> > In fact, _all_ hubs support changing the polling interval.  The value
> > given in the USB spec is just an upper limit; any smaller value is
> > equally acceptable.
> >
> > So why are you doing this only for hard-wired hubs?  Why not for all
> > hubs?
> 
> Because we only want to apply it to a specific device instance under
> our control.

Why?  What's so special about that device instance?

For example, why not instead have a poll_interval sysfs attribute for
all hubs that can be written from userspace?  Then people could reduce
the autoresume latency for any device they want.

> We apply autosuspend to built-in touchpad device for power savings,
> 
> Users can attach external hub devices with same VID:PID that we don't want to
> change the behavior.

Why don't you want to change the behavior?  Or allow the user to change 
the behavior?

>  Maybe disabling autosuspend for external HIDs
> can be more reasonable for that case?

If it makes sense to to save power for your built-in touchpad device, 
why doesn't it also make sense to save power for other external HIDs?

> > And is 250 ms really too long to wait for remote wakeup or connect
> > detection?  What's the real motivation behind this change?
> 
> When a user starts to move the cursor while touchpad is in autosuspend state,
> It takes more than >250ms (worst case can be >500ms) to wake up and response.
> That makes the cursor stuck for a while and warp to another location suddenly.

All right, that's a good reason.  But doesn't it apply just as well to 
other devices, not only your built-in touchpad?

Alan Stern