linux-kernel - Re: [PATCH] HID: usbhid: do not sleep when opening device

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Date:   Fri, 29 May 2020 18:09:26 -0700
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Guenter Roeck <groeck@...gle.com>
Cc:     Nicolas Boichat <drinkcat@...omium.org>,
        Jiri Kosina <jikos@...nel.org>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        Guenter Roeck <groeck@...omium.org>, linux-usb@...r.kernel.org,
        "open list:HID CORE LAYER" <linux-input@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] HID: usbhid: do not sleep when opening device

On Fri, May 29, 2020 at 05:48:26PM -0700, Guenter Roeck wrote:
> On Fri, May 29, 2020 at 4:50 PM Nicolas Boichat <drinkcat@...omium.org> wrote:
> >
> > On Sat, May 30, 2020 at 3:59 AM Dmitry Torokhov
> > <dmitry.torokhov@...il.com> wrote:
> > >
> > > usbhid tries to give the device 50 milliseconds to drain its queues
> > > when opening the device, but does it naively by simply sleeping in open
> > > handler, which slows down device probing (and thus may affect overall
> > > boot time).
> > >
> > > However we do not need to sleep as we can instead mark a point of time
> > > in the future when we should start processing the events.
> > >
> > > Reported-by: Nicolas Boichat <drinkcat@...omium.org>
> > > Signed-off-by: Dmitry Torokhov <dmitry.torokhov@...il.com>
> > > ---
> > >  drivers/hid/usbhid/hid-core.c | 27 +++++++++++++++------------
> > >  drivers/hid/usbhid/usbhid.h   |  1 +
> > >  2 files changed, 16 insertions(+), 12 deletions(-)
> > >
> > > diff --git a/drivers/hid/usbhid/hid-core.c b/drivers/hid/usbhid/hid-core.c
> > > index c7bc9db5b192..e69992e945b2 100644
> > > --- a/drivers/hid/usbhid/hid-core.c
> > > +++ b/drivers/hid/usbhid/hid-core.c
> > > @@ -95,6 +95,19 @@ static int hid_start_in(struct hid_device *hid)
> > >                                 set_bit(HID_NO_BANDWIDTH, &usbhid->iofl);
> > >                 } else {
> > >                         clear_bit(HID_NO_BANDWIDTH, &usbhid->iofl);
> > > +
> > > +                       if (test_and_clear_bit(HID_RESUME_RUNNING,
> > > +                                              &usbhid->iofl)) {
> > > +                               /*
> > > +                                * In case events are generated while nobody was
> > > +                                * listening, some are released when the device
> > > +                                * is re-opened. Wait 50 msec for the queue to
> > > +                                * empty before allowing events to go through
> > > +                                * hid.
> > > +                                */
> > > +                               usbhid->input_start_time = jiffies +
> > > +                                                          msecs_to_jiffies(50);
> > > +                       }
> > >                 }
> > >         }
> > >         spin_unlock_irqrestore(&usbhid->lock, flags);
> > > @@ -280,7 +293,8 @@ static void hid_irq_in(struct urb *urb)
> > >                 if (!test_bit(HID_OPENED, &usbhid->iofl))
> > >                         break;
> > >                 usbhid_mark_busy(usbhid);
> > > -               if (!test_bit(HID_RESUME_RUNNING, &usbhid->iofl)) {
> > > +               if (!test_bit(HID_RESUME_RUNNING, &usbhid->iofl) &&
> > > +                   time_after(jiffies, usbhid->input_start_time)) {
> >
> > Are we worried about jiffies overflowing (32-bit@...0Hz is "only" 49.7 days...)
> >
> 
> time_after() is overflow-safe. That is why it is used and jiffies is
> not compared directly.

Well, it is overflow safe, but still can not measure more than 50 days,
so if you have a device open for 50+ days there will be a 50msec gap
where it may lose events.

I guess we can switch to ktime(). A bit more expensive on 32 bits, but
in reality I do not think anyone would care.

Thanks.

-- 
Dmitry