On arm64, and possibly other architectures, requesting
IO coherent memory may return Normal-NC if the underlying
hardware isn't coherent. If these pages are then
remapped into userspace as Normal, that defeats the
purpose of getting Normal-NC, as well as resulting in
mappings with differing cache attributes.
In particular this happens with libusb, when it attempts
to create zero-copy buffers as is used by rtl-sdr, and
maybe other applications. The result is usually
application death.
If dma_mmap_attr() is used instead of remap_pfn_range,
the page cache/etc attributes can be matched between the
kernel and userspace.
Signed-off-by: Jeremy Linton <jeremy.linton@xxxxxxx>
---
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..1e7458dd6e5d 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_attrs(hcd->self.sysdev, vma, mem, dma_handle, size, 0)) {
dec_usb_memory_use_count(usbm, &usbm->vma_use_count);
return -EAGAIN;
}
--
2.24.1
