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

["Frank Schäfer" <fschaefer.oss@googlemail.com>]

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 ;) ).

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.

Merry Christmas !
Frank

>
> Regards,
> Mauro