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

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Message-ID: <CAATdQgBPrk=obCOiMAe1zAoP1As21MuzGzn-ixU56EmSkdQr1w@mail.gmail.com>
Date:   Tue, 19 Nov 2019 13:18:38 +0800
From:   Ikjoon Jang <ikjn@...omium.org>
To:     Alan Stern <stern@...land.harvard.edu>
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 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.
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. Maybe disabling autosuspend for external HIDs
can be more reasonable for that case?

>
> 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.

Thanks.
>
> Alan Stern
>