Hi Alan On Thu, 15 Dec 2022 at 16:34, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > > On Thu, Dec 15, 2022 at 11:57:17AM +0100, Ricardo Ribalda wrote: > > There is no need to make a kzalloc just for 16 bytes. Let's embed the data > > into the main data structure. > > > > Now that we are at it, lets remove all the castings and open coding of > > offsets for it. > > > > [Christoph, do you think dma wise we are violating any non written rules? :) thanks] > > There _is_ a rule, and it is not exactly unwritten. The kerneldoc for > the transfer_buffer member of struct urb says: > > This buffer must be suitable for DMA; allocate it with > kmalloc() or equivalent. > > Which in general means that the buffer must not be part of a larger > structure -- not unless the driver can guarantee that the structure will > _never_ be accessed while a USB transfer to/from the buffer is taking > place. > Thanks a lot for the clarification. I was mainly looking at the kerneldoc from: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/usb.h#n1687 and I could not see any reference to the DMA requirements. Mind if I send a patch to add a reference there? > There are examples all over the USB subsystem where buffers as small as > one or two bytes get kmalloc'ed in order to obey this rule. 16 bytes is > certainly big enough that you shouldn't worry about it being allocated > separately. > Yep, we should keep it malloced. Thanks a lot for looking into this :) > Alan Stern -- Ricardo Ribalda
