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    SubjectRe: Linux UIO driver cache problem in PowerPC (fix)
    Am Thu, 13 Mar 2008 08:19:32 +0100
    schrieb Hans-Jürgen Koch <hjk@linutronix.de>:

    > Am Wed, 12 Mar 2008 15:22:59 -0400
    > schrieb "Jean-Samuel Chenard" <jsamch@gmail.com>:
    >
    > > Hi,

    Hi Jean-Samuel,
    I investigated a bit and found your patch to be correct. I cleaned it
    up a bit and sent it to Greg and LKML. I added you as author and also
    took the freedom to add your Signed-off-by.

    Juergen, Thomas, Grant, if you can confirm this, please add your
    signature as well.

    Thanks,
    Hans

    >
    > 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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