[RFC] mmaping with VIVT cache

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

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.

I've tried to introduce an API for mapping "coherent" DMA buffers
into userspace, and it got nowhere (well, only ARM implements it,
and hardly anyone uses it) - hence why we still have these problems.

So I don't have an answer to solve your problem - afaiac, it's up to
the device driver authors to sort it out, and apply pressure to other
parts of the kernel community (those who resist a way to make this
kind of thing work) to sort out how to cleanly map buffers used for
DMA into userspace.