On Fri, May 01, 2020 at 10:47:22AM -0500, Jeremy Linton wrote: > Hi, > > Thanks for taking a look at this. > > On 5/1/20 2:05 AM, Greg KH wrote: > > On Thu, Apr 30, 2020 at 04:19:22PM -0500, Jeremy Linton wrote: > > > 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. > > > > What is "Normal-NC"? > > A non-cacheable attribute on arm64 pages. I think Mark R & Marc Z elaborated > while I was asleep (thanks!). > . > > > > > > > In particular this happens with libusb, when it attempts > > > to create zero-copy buffers as is used by rtl-sdr, and > > > > What is "rtl-sdr" > > Its the realtek software defined radio (SDR), a really inexpensive TV dongle > that was discovered could be used as a general purpose SDR a decade or so > ago. In particular, this project > https://github.com/osmocom/rtl-sdr/ > which is packaged by fedora/etc. > > > > > > maybe other applications. The result is usually > > > application death. > > > > So is this a new problem? Old problem? Old problem only showing up on > > future devices? On current devices? I need a hint here as to know if > > this is a bugfix or just work to make future devices work properly. > > This has been a problem on arm devices without IO coherent USB apparently > for years. The rtl-sdr project itself has a disable zero-copy mode that > people have been using on rpi/etc specific builds. Fedora OTOH, is building > it with the same flags on x86 & arm64 which means that people report > problems. This happened a few days ago (on a pinebook), and I duplicated it > on an NXP platform just running the `rtl_test` artifact with a nooelec from > my junk box. Guessing that it was a page mismatch I went looking for that, > rather than disabling the zero copy since punishing arm machine that have IO > coherent USB adapters for the sins of these low end devices isn't ideal. I > found this, and this patch allows the rtl_test app to run without issues on > my NXP/solidrun. > > Plus, given that its actually a kernel/libusb problem its likely there are > other applications having similar problems. > > > > > > > > > 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)) { > > > > Given that this code has not changed since 2016, how has no one noticed > > this issue before? > They have there are a lot of reports of sdr failures, but the general use > case is rare? > > > > > And have you tested this change out on other systems (i.e. x86) to > > ensure that this still works properly? > > Yes and no, I did some basic libusb tests on an x86 machine, but its a bit > tricky at the moment for me to get the rtl plugged into a x86 test machine. > (its a work in progress). > > > > > > And why isn't this call used more by drivers if this is a real issue? > The particulars of asking for iocoherent memory and then mapping it to > userspace is rarer than just asking for kmalloc()/remap() and then > performing the dma ops? > > Then there are all the softer issues around arm64 testing/availability and > vendors carrying "fixes" for particular issues (like rtl-sdr disabling zero > copy). > > > And will this cause issues with how the userspace mapping is handled as > > now we rely on userspace to do things differently? Or am I reading the > > dma_mmap_attrs() documentation wrong? > I don't think userspace is doing anything differently here, and AFAIK, on > systems with IO coherent adapters this ends up with the same page mapping as > just doing the remap_pfn_rage() with the same attributes as before. I've > looked at dma_map_attrs() a bit, but i'm also trusting it does what it says > on the tin. > > > Thanks again for looking at this. Ok, can I get a lot better written changelog text for this patch based on this thread, so that it makes more sense when we merge this patch? thanks, greg k-h
