Re: [PATCHv2] usb: gadget: get rid of USB_GADGET_DUALSPEED andUSB_GADGET_SUPERSPEED

[Michal Nazarewicz]

On Thu, 18 Aug 2011 16:59:28 +0200, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>  
wrote:

> On Thu, 18 Aug 2011, Michal Nazarewicz wrote:
>
> > On Wed, 17 Aug 2011 23:09:37 +0200, Alan Stern  
> > <stern@xxxxxxxxxxxxxxxxxxx>
> > wrote:
> > > I see what's going on here.  Your original patch was wrong and then my
> > > correction was wrong as well.
> > >
> > > This line has to remain the way it was (although those (u8) typecasts
> > > don't seem to be necessary).  Above, you have to initialize
> > > composite_driver.speed to an appropriate value, probably
> > > USB_SPEED_SUPER.
> > >
> > > What you didn't realize in your original patch is that
> > > usb_composite_probe() gets called more than once.  Each time it is
> > > called, it has to adjust composite_driver.speed.
> >
> > That's sneaky of composite.c...  But is it desired behaviour?
>
> Yes, it is.
>
> >  I cannot
> > came up with a situation where that's what we want.  I would imagine  
> > that
> > usb_composite_probe() should assume the same speed for given
> > usb_composite_driver regardless if some other slower  
> > usb_composite_driver
> > was loaded.
>
> The speed being calculated isn't the speed of the usb_composite_driver;
> it's the speed of the usb_gadget_driver.  The gadget itself cannot be
> allowed to run faster than its internal drivers can handle.

Yeah, that's why you set usb_gadget_driver's speed to the speed declared
in usb_composite_driver.

For the most part, usb_composite_probe() is called only once in module's
init function.  As far as I know, only g_ffs calls it several times.  So
in all cases expect for g_ffs, composite_driver.speed =  
min(composite_driver.speed,
driver->max_speed) should have the same effect as composite_driver.speed
= driver->max_speed.

> For example, if you have a composite gadget where one of the function
> drivers can handle SuperSpeed and the other can't go beyond high speed,
> the overall gadget must never run faster than high speed.

Shouldn't that be dealt in usb_add_function()?  I cannot see any code that
would do that here atm though.

--
Best regards,                                         _     _
.o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
..o | Computer Science,  Michal "mina86" Nazarewicz    (o o)
ooo +-----<email/xmpp: mnazarewicz@xxxxxxxxxx>-----ooO--(_)--Ooo--
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/