Re: [PATCH 6/6] usb: musb: Decrease URB starting latency in musb_advance_schedule()

[Matwey V. Kornilov]

ÐÑ, 30 ÐÐÑ. 2019 Ð. Ð 18:31, Bin Liu <b-liu@xxxxxx>:
>
> Hi Greg and all devs,
>
> On Wed, Apr 03, 2019 at 09:53:10PM +0300, Matwey V. Kornilov wrote:
> > Previously, the algorithm was the following:
> >
> >  1. giveback current URB
> >  2. if current qh is not empty
> >     then start next URB
> >  3. if current qh is empty
> >     then dispose the qh, find next qh if any, and start URB.
> >
> > It may take a while to run urb->callback inside URB giveback which is
> > run synchronously in musb. In order to improve the latency we rearrange
> > the function behaviour for the case when qh is not empty: next URB is
> > started before URB giveback. When qh is empty then the behaviour is
> > intentionally kept in order not to break existing inter qh scheduling:
> > URB giveback could potentionally enqueue other URB to the empty qh
> > preventing it from being disposed.
>
> This patch changes the sequence of urb giveback in musb.
>
>         before                          after
>         ------                          -----
> 1. giveback current urb                 1. start next urb if qh != empty
> 2. start next urb if qh != empty        2. giveback current urb
>
> I see there is a potential that the urb giveback could be out of order,
> for example, if urb giveback in BH and the next urb finishes before BH
> runs.

Could you please give more details? Frankly speaking, I am not sure
that I understand the reordering issue origin correctly.
I see in the existing implementation that the function call order is
the following:

1. glue interrupt handler (for instance dsps_interrupt() in my am335x
case) holds musb->lock;
2. musb_interrupt()
3. musb_host_rx() (or *_tx())
4. musb_advance_schedule()
5. musb_giveback() releases and reacquires musb->lock around:
6. usb_hcd_giveback_urb()

So, when musb_giveback() is called inside musb_advance_schedule() then
the second instance of musb_advance_schedule() can be started
simultaneously when the following interrupt is being handled at other
CPU core. And we can see two usb_hcd_giveback_urb() running
concurrently.
Is it correct?

>
> If this potential is possible, is it a problem for any class driver?
>
> Thanks,
> -Bin.
>
> >
> > Before this patch, time spent in urb->callback led to the following
> > glitches between the host and a hub during isoc transfer (line 4):
> >
> >     11.624492 d=  0.000124 [130.6 +  1.050] [  4] SPLIT
> >     11.624492 d=  0.000000 [130.6 +  1.467] [  3] IN   : 3.5
> >     11.624493 d=  0.000000 [130.6 +  1.967] [ 37] DATA0: aa 08 [skipped...]
> >     11.625617 d=  0.001124 [131.7 +  1.050] [  4] SPLIT
> >     11.625617 d=  0.000000 [131.7 +  1.467] [  3] IN   : 3.5
> >     11.625867 d=  0.000250 [132.1 +  1.050] [  4] SPLIT
> >     11.625867 d=  0.000000 [132.1 +  1.467] [  3] IN   : 3.5
> >     11.625868 d=  0.000001 [132.1 +  1.983] [  3] DATA0: 00 00
> >     11.626617 d=  0.000749 [132.7 +  1.050] [  4] SPLIT
> >     11.626617 d=  0.000000 [132.7 +  1.467] [  3] IN   : 3.5
> >     11.626867 d=  0.000250 [133.1 +  1.050] [  4] SPLIT
> >     11.626867 d=  0.000000 [133.1 +  1.467] [  3] IN   : 3.5
> >     11.626868 d=  0.000000 [133.1 +  1.967] [  3] DATA0: 00 00
> >
> > After the hub, they look as the following and may lead to broken
> > perepherial transfer (as in case of PWC based webcam):
> >
> >     11.332004 d=  0.000997 [ 30.0 +  3.417] [  3] IN   : 5.5
> >     11.332007 d=  0.000003 [ 30.0 +  6.833] [800] DATA0: 8a 1c [skipped...]
> >     11.334004 d=  0.001997 [ 32.0 +  3.417] [  3] IN   : 5.5
> >     11.334007 d=  0.000003 [ 32.0 +  6.750] [  3] DATA0: 00 00
> >     11.335004 d=  0.000997 [ 33   +  3.417] [  3] IN   : 5.5
> >     11.335007 d=  0.000003 [ 33   +  6.750] [  3] DATA0: 00 00
> >
> > Removing this glitches makes us able to successfully run 10fps
> > video stream from the webcam attached via USB hub. That was
> > previously impossible.
> >
> > Signed-off-by: Matwey V. Kornilov <matwey@xxxxxxxxxx>
> > ---
> >  drivers/usb/musb/musb_host.c | 18 ++++++++++++++++++
> >  1 file changed, 18 insertions(+)
> >
> > diff --git a/drivers/usb/musb/musb_host.c b/drivers/usb/musb/musb_host.c
> > index ed99ecd4e63a..75be92873b5b 100644
> > --- a/drivers/usb/musb/musb_host.c
> > +++ b/drivers/usb/musb/musb_host.c
> > @@ -85,6 +85,11 @@ static bool musb_qh_empty(struct musb_qh *qh)
> >       return list_empty(&qh->hep->urb_list);
> >  }
> >
> > +static bool musb_qh_singular(struct musb_qh *qh)
> > +{
> > +     return list_is_singular(&qh->hep->urb_list);
> > +}
> > +
> >  static void musb_qh_unlink_hep(struct musb_qh *qh)
> >  {
> >       if (!qh->hep)
> > @@ -362,6 +367,19 @@ static void musb_advance_schedule(struct musb *musb, struct urb *urb,
> >               break;
> >       }
> >
> > +     if (ready && !musb_qh_singular(qh)) {
> > +             struct urb *next_urb = list_next_entry(urb, urb_list);
> > +
> > +             musb_dbg(musb, "... next ep%d %cX urb %p", hw_ep->epnum, is_in ? 'R' : 'T', next_urb);
> > +             musb_start_urb(musb, is_in, qh, next_urb);
> > +
> > +             qh->is_ready = 0;
> > +             musb_giveback(musb, urb, status);
> > +             qh->is_ready = ready;
> > +
> > +             return;
> > +     }
> > +
> >       qh->is_ready = 0;
> >       musb_giveback(musb, urb, status);
> >       qh->is_ready = ready;
> > --
> > 2.16.4
> >



-- 
With best regards,
Matwey V. Kornilov.
Sternberg Astronomical Institute, Lomonosov Moscow State University, Russia
119234, Moscow, Universitetsky pr-k 13, +7 (495) 9392382