[RFC] mmaping with VIVT cache

[Russell King - ARM Linux]

On Wed, Sep 15, 2010 at 09:06:02AM +0800, Eric Miao wrote:
> On Wed, Sep 15, 2010 at 7:50 AM, Russell King - ARM Linux
> <linux at arm.linux.org.uk> wrote:
> > On Mon, Sep 13, 2010 at 12:58:41PM -0700, P F wrote:
> >> Hi,
> >>
> >> Please excuse the pseudonym.
> >>
> >> I have recently encountered a problem in the context of the UVC (webcam)
> >> driver.  The driver maintainer, Laurent, suggested that we bring the
> >> thread to linux-arm-kernel for wider discussion.  See here for reference:
> >> http://lists.berlios.de/pipermail/linux-uvc-devel/2010-August/005864.html
> >>
> >> The problem I encountered was this: using an ARM926EJ-S with OHCI USB host,
> >> I noticed intermittent graphical corruption of frames captured by several
> >> different webcams, both UVC and non-UVC.
> >>
> >> I was able to isolate the corruption down to one specific instance.  In
> >> this instance, I collected three images:
> >> 1A.jpg - collected from host, prior to corruption
> >> 2A.jpg - collected from host, demonstrating corruption
> >> 2B.jpg - collected from inline USB analyzer, same image as 2A
> >>
> >> Images 1A and 2A are in sequence, i.e., 2A immediately follows 1A.  2B is
> >> not corrupted, and a binary diff between 2A and 2B shows two 32-byte
> >> chunks have been changed.  A binary diff between 2A and 1A shows that
> >> these same two chunks match.  Therefore, it seems there is some
> >> contamination of the second image by the first.
> >>
> >> As I described in the linked message, I found a workaround for this issue
> >> by adding
> >>   vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot)
> >> in the UVC driver's mmap() function, but Laurent explained that this was
> >> invalid.
> >>
> >> Does anybody have ideas for a proper fix?  I can reproduce this corruption
> >> easily, and would be happy to test proposed patches.
> >
> > The fact is that mapping buffers as cacheable into userspace on non-
> > coherent architectures _will_ result in the cache contents getting in
> > the way of updates to the DMA'd buffer.  It's a fundamental property
> > of such hardware implementations.
> >
> 
> Maybe we can just detect if the buffer is mapped in alias, and modify the
> PTE, just as what we did to inconsistent write-combine? So to make this
> transparent to other consistent architectures?

All pages which are mapped into userspace have at least one alias.