Em Wed, 02 Oct 2013 12:38:14 -0400 (EDT) Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> escreveu: > On Wed, 2 Oct 2013, Mauro Carvalho Chehab wrote: > > > > > So, there's no need to call usb_change_ep_bandwidth(). > > > > > > That's right. > > > > > > > If so, then usb_change_ep_bandwidth() as a quirk, if bInterval > > > > or wMaxPacketSize were improperly filled. > > > > > > > > Right? > > > > > > Or if the values are correct, but the driver wants to use something > > > different for its own reasons (for example, to get lower latency or > > > because it knows that it will never use packets as large as the > > > descriptor allows). Right. > > > > Ok, so, in this case, usb_change_ep_bandwidth() could be called > > just before usb_alloc_urb(), in order to make it to use the packet > > size that would be expected for that kind of ISOC traffic that > > userspace indirectly selected, by adjusting the streaming > > video resolution selected, right? > > We haven't decided on the final API yet. However, note that > usb_alloc_urb() doesn't depend on the packet size. It requires you to > specify only the number of packets, not their sizes. Therefore it > doesn't matter whether you call usb_change_ep_bandwidth() before or > after usb_alloc_urb(). Sure, but, at least on almost all V4L2 drivers, the number of packets and their sizes should be calculated to be able to receive all URBs needed to store a complete image frame (that generally arrives on every 1/60 Hz or 1/50 Hz - depending on the frames per second rate). On those drivers, the transfer_buffer is allocated at the same loop where usb_alloc_urb() is called. So, it makes sense for them to specify the bandwidth parameters at the function that calls usb_alloc_coherent() and usb_alloc_urb(). Regards, Mauro -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
