Messages in this thread | | | Date | Thu, 13 Mar 2008 08:19:32 +0100 | From | Hans-Jürgen Koch <> | Subject | Re: Linux UIO driver cache problem in PowerPC (fix) |
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Am Wed, 12 Mar 2008 15:22:59 -0400 schrieb "Jean-Samuel Chenard" <jsamch@gmail.com>:
> Hi,
Hi Jean-Samuel, thanks for your report. Please CC LKML if you find bugs like this. I also added Grant Likely (PPC Xilinx Virtex maintainer), as it seems to be a platform specific problem.
> > Experimenting with your userspace I/O driver on a Xilinx FPGA (PowerPC > 405), I was having problems when reading hardware device registers > mapped on my platform using the user-space mmap() call. > > After some investigation (looking at the driver/char/mspec.c file), I > found that preventing caching of the page in uio_mmap_physical fixed > my issues with userspace mmap() reading messed-up values.
Hm, we already mark the page with VM_IO. That's sufficient on x86 and arm. I'm not sure whether this is a bug in PPC memory handling. Why do they cache VM_IO pages?
> > This patch is against your initial commit of the UIO driver in the > mainline kernel tree. > > ==== > diff --git a/drivers/uio/uio.c b/drivers/uio/uio.c > index 865f32b..36e1123 100644 > --- a/drivers/uio/uio.c > +++ b/drivers/uio/uio.c > @@ -22,6 +22,7 @@ > #include <linux/string.h> > #include <linux/kobject.h> > #include <linux/uio_driver.h> > +#include <asm/pgtable.h> > > #define UIO_MAX_DEVICES 255 > > @@ -447,6 +448,8 @@ static int uio_mmap_physical(struct > vm_area_struct *vma) > > vma->vm_flags |= VM_IO | VM_RESERVED; > > + vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot); > + > return remap_pfn_range(vma, > vma->vm_start, > idev->info->mem[mi].addr >> PAGE_SHIFT, > ==== > > I am a bit unsure if this will break something in any of your uses of > the UIO driver (on other platforms than ppc), but it really fixes > things for me.
It should be OK, but unneccesary on other platforms. I have no objections, but I fear in 6 months we'll see a patch removing that line again because it's not needed...
> Let me know if you need more details on what was > happening on my platform before I added this statement.
I'd like to hear the opinion of people really involved in PPC memory handling.
> > Thanks for the great driver for user-space I/O, it will save me lots > of time in my research. > > Regards, > > Jean-Samuel
Thanks, Hans
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