usb zero copy dma handling

[yvahknez.7f7d2ff0@hashmail.org]

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Hello linux-arm.

this is a followup to that thread from 2018-11:
https://www.spinics.net/lists/arm-kernel/msg685598.html

the issue was discussed in more detail than i can claim
to fully understand back then, but no fix ever merged.
but i would really like to use rtl_433 on a raspi without
having to build a custom-patched kernel first.

the attached patch is my stripdown/cleanup of a devel-diff
provided to me by the original reporter Steve Markgraf.
credits to him for the good parts, blame to me for the bad parts.

this does not cover the additional case of "PIO-based usb controllers"
mainly because i dont understand what that means (or how to handle it)
and if its broken right now (as the thread indicates) it might
as well stay broken until someone who understands cares enough.

could you please get this on track for merging?

regards,
  x23



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diff --git a/drivers/usb/core/devio.c b/drivers/usb/core/devio.c
index ffccd40ea67d..0fedf4a97f65 100644
--- a/drivers/usb/core/devio.c
+++ b/drivers/usb/core/devio.c
@@ -240,9 +240,14 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma)
 	usbm->vma_use_count = 1;
 	INIT_LIST_HEAD(&usbm->memlist);
 
+#ifdef CONFIG_X86
 	if (remap_pfn_range(vma, vma->vm_start,
 			virt_to_phys(usbm->mem) >> PAGE_SHIFT,
 			size, vma->vm_page_prot) < 0) {
+#else /* !CONFIG_X86 */
+	if (dma_mmap_coherent(ps->dev->bus->sysdev, 
+			vma, mem, dma_handle, size) < 0) {
+#endif /* !CONFIG_X86 */
 		dec_usb_memory_use_count(usbm, &usbm->vma_use_count);
 		return -EAGAIN;
 	}

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