Linux UIO driver cache problem in PowerPC (fix)
grant.likely at secretlab.ca
Fri Mar 14 08:53:58 EST 2008
On Thu, Mar 13, 2008 at 1:19 AM, Hans-Jürgen Koch <hjk at linutronix.de> wrote:
> Am Wed, 12 Mar 2008 15:22:59 -0400
> schrieb "Jean-Samuel Chenard" <jsamch at 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.
Hmmm. I'm not a memory handling expert so I don't know what's going
on here. Josh, does this look like anything that would be ppc405
> > 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
> > Thanks for the great driver for user-space I/O, it will save me lots
> > of time in my research.
> > Regards,
> > Jean-Samuel
Grant Likely, B.Sc., P.Eng.
Secret Lab Technologies Ltd.
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