On Wed, 2 Jul 2014, Peter Stuge wrote: > > The kernel doesn't support scatter-gather for control transfers, only > > bulk. > > That could possibly change, right, and then it would be nice to have > zerocopy for free there as well? No. ohci-hcd doesn't support control transfers larger than 4 KB anyway, and ehci-hcd doesn't support control transfers larger than 16 KB. This is not likely to change. Besides, a control transfer can never be larger than 64 KB, and throughput isn't an issue for them. > > > Indeed I think userspace wants to be involved in choosing memory also > > > with bulk, in order to ensure that zerocopy will always work when > > > userspace cares about that. > > > > > > Is it enough to expose the DMA mask of the host controller? > > > > It doesn't need to be exposed, since the mmap(2) call would be handled > > by the kernel's USB stack (and besides, the user program can't request > > that the mapped memory be located in any particular physical address > > region). > > Since alignment isn't the only issue I don't think there's a way to > avoid it. I was just hoping to be able to avoid allocating zerocopy > buffers with mmap(). How else can you guarantee that a buffer is located in the first 4 GB of memory? Even on a 32-bit system (if the computer has more than 4 GB of RAM)? Alan Stern -- 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
