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

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Message-ID: <Pine.LNX.4.44L0.1911191008440.1506-100000@iolanthe.rowland.org>
Date:   Tue, 19 Nov 2019 10:14:27 -0500 (EST)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Ikjoon Jang <ikjn@...omium.org>
cc:     linux-usb@...r.kernel.org, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Suwan Kim <suwan.kim027@...il.com>,
        "Gustavo A . R . Silva" <gustavo@...eddedor.com>,
        Johan Hovold <johan@...nel.org>,
        Nicolas Boitchat <drinkcat@...omium.org>
Subject: Re: [PATCH 0/2] usb: override hub device bInterval with device node

On Tue, 19 Nov 2019, Ikjoon Jang wrote:

> On Sun, Nov 17, 2019 at 11:46 PM Alan Stern <stern@...land.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