On some architectures (e.g. arm64) requests for IO coherent memory may use non-cachable attributes if the relevant device isn't cache coherent. If these pages are then remapped into userspace as cacheable, they may not be coherent with the non-cacheable mappings. In particular this happens with libusb, when it attempts to create zero-copy buffers for use by rtl-sdr (https://github.com/osmocom/rtl-sdr/). On low end arm devices with non-coherent USB ports, the application will be unexpectedly killed, while continuing to work fine on arm machines with coherent USB controllers. This bug has been discovered/reported a few times over the last few years. In the case of rtl-sdr a compile time option to enable/disable zero copy was implemented to work around it. Rather than relaying on application specific workarounds, dma_mmap_coherent() can be used instead of remap_pfn_range(). The page cache/etc attributes will then be correctly set in userspace to match the kernel mapping. Signed-off-by: Jeremy Linton <jeremy.linton@xxxxxxx> --- v1->v2: Update commit message and change to dma_mmap_coherent() from dma_mmap_attr(,,,0) which are the same. drivers/usb/core/devio.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-) diff --git a/drivers/usb/core/devio.c b/drivers/usb/core/devio.c index 6833c918abce..b9db9812d6c5 100644 --- a/drivers/usb/core/devio.c +++ b/drivers/usb/core/devio.c @@ -217,6 +217,7 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma) { struct usb_memory *usbm = NULL; struct usb_dev_state *ps = file->private_data; + struct usb_hcd *hcd = bus_to_hcd(ps->dev->bus); size_t size = vma->vm_end - vma->vm_start; void *mem; unsigned long flags; @@ -250,9 +251,7 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma) usbm->vma_use_count = 1; INIT_LIST_HEAD(&usbm->memlist); - if (remap_pfn_range(vma, vma->vm_start, - virt_to_phys(usbm->mem) >> PAGE_SHIFT, - size, vma->vm_page_prot) < 0) { + if (dma_mmap_coherent(hcd->self.sysdev, vma, mem, dma_handle, size)) { dec_usb_memory_use_count(usbm, &usbm->vma_use_count); return -EAGAIN; } -- 2.24.1
