Hi, On Tue, Aug 23, 2011 at 04:49:37PM -0400, Alan Stern wrote: > On Tue, 23 Aug 2011, Michal Nazarewicz wrote: > > > On Tue, 23 Aug 2011 20:00:25 +0200, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> > > wrote: > > > For the time being, perhaps the best answer is to use the _lowest_ > > > max_speed value among all the function drivers. It's the simplest > > > answer and most likely to be correct most of the time. > > > > In theory, one could create a composite function with only SS/HS > > descriptors. > > The only reason for doing that would be if the function was incapable > of carrying out its duties when running at full speed (insufficient > bandwidth, for example). Something like this is discussed briefly in > section 9.2.6.6 of the USB-2 spec. > > > So it would work for SS/HS but not for FS. I'm not > > saying that it would make sense (or that it's not a bug) but that > > adds to the discussion. > > If somebody writes a composite gadget containing two functions, one of > which supports only full speed and the other only high speed, they'll > get what they deserve. :-) > > Still, maybe I was wrong. Maybe it would be better to use the fastest > speed supported by at least one of the function drivers. The user can > always force a SuperSpeed-capable device to run at high speed by using > a USB-2 cable to plug it in. I'm not sure whether the user could force > such a device to run at full speed, however. plug it behind a full speed hub, like those that usually come on USB keyboards. -- balbi
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