Re: [PATCH v2 21/21] em28xx: add module parameter for selection of the preferred USB transfer type

[Mauro Carvalho Chehab <mchehab@redhat.com>]

Em Sun, 23 Dec 2012 15:01:26 +0100
Frank Schäfer <fschaefer.oss@googlemail.com> escreveu:

> Am 23.12.2012 14:44, schrieb Mauro Carvalho Chehab:
> > Hi Frank,
> >
> > Em Thu,  8 Nov 2012 20:11:53 +0200
> > Frank Schäfer <fschaefer.oss@googlemail.com> escreveu:
> >
> >> By default, isoc transfers are used if possible.
> >> With the new module parameter, bulk can be selected as the
> >> preferred USB transfer type.
> > I did some tests yesterday with prefer_bulk. IMHO, webcams should
> > select bulk mode by default, as this allows more than one camera to
> > work at the same time (I tested yesterday with 3 Silvercrest ones on
> > my notebook). With ISOC transfers, the core won't let it to happen, as
> > a single camera reserves 51% of the max allowed isoc traffic.
> 
> Ok. I just didn't want to change the current behavior because of
> potential regressions.
> Why not change it for all devices ? Frame data processing with bulk
> transfers has a smaller overhead than with isoc (although not really
> measurable ;) ).

It is better to keep it as-is for the other devices. There are simply too
much non-webcam devices for us to be sure that this will always work. 
As there are very few webcams supported, the risk of this change is low
if applied only to webcams.

> I will send a patch after christmas.
> 
> >
> >> Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
> >> ---
> >>  drivers/media/usb/em28xx/em28xx-cards.c |   11 +++++++++--
> >>  1 Datei geändert, 9 Zeilen hinzugefügt(+), 2 Zeilen entfernt(-)
> >>
> >> diff --git a/drivers/media/usb/em28xx/em28xx-cards.c b/drivers/media/usb/em28xx/em28xx-cards.c
> >> index a9344f0..7f5b303 100644
> >> --- a/drivers/media/usb/em28xx/em28xx-cards.c
> >> +++ b/drivers/media/usb/em28xx/em28xx-cards.c
> >> @@ -61,6 +61,11 @@ static unsigned int card[]     = {[0 ... (EM28XX_MAXBOARDS - 1)] = UNSET };
> >>  module_param_array(card,  int, NULL, 0444);
> >>  MODULE_PARM_DESC(card,     "card type");
> >>  
> >> +static unsigned int prefer_bulk;
> >> +module_param(prefer_bulk, int, 0644);
> > This needs to be changed to 0444, as prefer_bulk doesn't allow changing
> > it dynamically, as the test is done during device probe, not at stream on.
> 
> Good catch !
> Can you fix it ? I'm a bit in hurry right now.
> Otherwise I will try to send a patch tomorrow.

Yeah, I can do it.

> Merry Christmas !
> Frank

Merry Christmas!

-- 

Cheers,
Mauro