On Tue, Jun 22, 2021 at 08:07:12AM +0000, David Laight wrote: > From: Mauro Carvalho Chehab > > Sent: 21 June 2021 14:40 > > > > As warned by smatch: > > drivers/media/usb/uvc/uvc_v4l2.c:911 uvc_ioctl_g_input() error: doing dma on the stack (&i) > > drivers/media/usb/uvc/uvc_v4l2.c:943 uvc_ioctl_s_input() error: doing dma on the stack (&i) > > > > those two functions call uvc_query_ctrl passing a pointer to > > a data at the DMA stack. those are used to send URBs via > > usb_control_msg(). Using DMA stack is not supported and should > > not work anymore on modern Linux versions. > > > > So, use a kmalloc'ed buffer. > ... > > + buf = kmalloc(1, GFP_KERNEL); > > + if (!buf) > > + return -ENOMEM; > > + > > ret = uvc_query_ctrl(chain->dev, UVC_GET_CUR, chain->selector->id, > > chain->dev->intfnum, UVC_SU_INPUT_SELECT_CONTROL, > > - &i, 1); > > + buf, 1); > > Thought... > > Is kmalloc(1, GFP_KERNEL) guaranteed to return a pointer into > a cache line that will not be accessed by any other code? > (This is slightly weaker than requiring a cache-line aligned > pointer - but very similar.) As I understand it, on architectures that do not have cache-coherent I/O, kmalloc is guaranteed to return a buffer that is cacheline-aligned and whose length is a multiple of the cacheline size. Now, whether that buffer ends up being accessed by any other code depends on what your driver does with the pointer it gets from kmalloc. :-) Alan Stern > Without that guarantee you can't use the returned buffer for > read dma unless the memory accesses are coherent. > > David
