On Wed, 6 Jan 2016, Steinar H. Gunderson wrote: > On Tue, Jan 05, 2016 at 10:49:49PM -0800, Christoph Hellwig wrote: > > This is a completely broken usage of the mmap interface. if you use > > mmap on a device file you must use the actual mmap for the data > > transfer. > > Really? V4L does exactly the same thing, from what I can see. It's just a way > of allocating memory with specific properties, roughly similar to hugetlbfs. > > > Our interface for zero copy reads/writes is O_DIRECT, and that requires > > not special memory allocation, just proper alignment. > > But that assumes you are using I/O using read()/write(). There's no way you > can shoehorn USB isochronous reads into the read() interface, O_DIRECT or not. Indeed, the I/O operations we are using with mmap here are not reads or writes; they are ioctls. As far as I know, the kernel doesn't have any defined interface for zerocopy ioctls. Furthermore, this approach _does_ use the mmap for data transfers. I'm not sure what Christoph was referring to. 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
