On Tue, Apr 5, 2011 at 4:18 PM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > On Tue, 5 Apr 2011, Mian Yousaf Kaukab wrote: > >> Instead of removing short_not_ok can we use something like following? >> bh->outreq->short_not_ok = (amount % maxp) ? 0 : 1; >> short_not_ok is useful for example when it is not possible to do DMA >> tranfers from usb controller's buffer to the memory for unaligned >> lengths. >> controller driver can decide to use dma or pio mode based on if short >> packets are expected or not. > > You are trying to use the short_not_ok flag for something it wasn't > designed for. Its real purpose is to allow drivers to delimit > consecutive bulk transfers, in the case where the first transfer ends > early. > > Besides, what is your controller going to do if a full-sized packet is > expected but a short packet actually arrives? AFAIK, only mass storage class sets this flag in the kernel and in mass-storage it is already a protocol error if a short-packet arrives when a maxp size packet was expected. If we don't use short_not_ok, then we will have to wait for each packet to arrive and then based on its size we can set dma. This is what we do if short_not_ok is set to 0. if short_not_ok=1, we assume that protocol is ensuring no short packets so we optimize dma setup based on this. > > Alan Stern > > Thanks, -- Mian Yousaf Kaukab -- 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
